GIT-FETCH(1) - phpMan

GIT-FETCH(1) Git Manual GIT-FETCH(1)

NAME
git-fetch - Download objects and refs from another repository

SYNOPSIS
git fetch [<options>] [<repository> [<refspec>...]]
git fetch [<options>] <group>
git fetch --multiple [<options>] [(<repository> | <group>)...]
git fetch --all [<options>]

DESCRIPTION
Fetch branches and/or tags (collectively, "refs") from one or more other repositories,
along with the objects necessary to complete their histories. Remote-tracking branches are
updated (see the description of <refspec> below for ways to control this behavior).

By default, any tag that points into the histories being fetched is also fetched; the
effect is to fetch tags that point at branches that you are interested in. This default
behavior can be changed by using the --tags or --no-tags options or by configuring
remote.<name>.tagOpt. By using a refspec that fetches tags explicitly, you can fetch tags
that do not point into branches you are interested in as well.

git fetch can fetch from either a single named repository or URL, or from several
repositories at once if <group> is given and there is a remotes.<group> entry in the
configuration file. (See git-config(1)).

When no remote is specified, by default the origin remote will be used, unless there's an
upstream branch configured for the current branch.

The names of refs that are fetched, together with the object names they point at, are
written to .git/FETCH_HEAD. This information may be used by scripts or other git commands,
such as git-pull(1).

OPTIONS
--all
Fetch all remotes.

-a, --append
Append ref names and object names of fetched refs to the existing contents of
.git/FETCH_HEAD. Without this option old data in .git/FETCH_HEAD will be overwritten.

--atomic
Use an atomic transaction to update local refs. Either all refs are updated, or on
error, no refs are updated.

--depth=<depth>
Limit fetching to the specified number of commits from the tip of each remote branch
history. If fetching to a shallow repository created by git clone with --depth=<depth>
option (see git-clone(1)), deepen or shorten the history to the specified number of
commits. Tags for the deepened commits are not fetched.

--deepen=<depth>
Similar to --depth, except it specifies the number of commits from the current shallow
boundary instead of from the tip of each remote branch history.

--shallow-since=<date>
Deepen or shorten the history of a shallow repository to include all reachable commits
after <date>.

--shallow-exclude=<revision>
Deepen or shorten the history of a shallow repository to exclude commits reachable
from a specified remote branch or tag. This option can be specified multiple times.

--unshallow
If the source repository is complete, convert a shallow repository to a complete one,
removing all the limitations imposed by shallow repositories.

If the source repository is shallow, fetch as much as possible so that the current
repository has the same history as the source repository.

--update-shallow
By default when fetching from a shallow repository, git fetch refuses refs that
require updating .git/shallow. This option updates .git/shallow and accept such refs.

--negotiation-tip=<commit|glob>
By default, Git will report, to the server, commits reachable from all local refs to
find common commits in an attempt to reduce the size of the to-be-received packfile.
If specified, Git will only report commits reachable from the given tips. This is
useful to speed up fetches when the user knows which local ref is likely to have
commits in common with the upstream ref being fetched.

This option may be specified more than once; if so, Git will report commits reachable
from any of the given commits.

The argument to this option may be a glob on ref names, a ref, or the (possibly
abbreviated) SHA-1 of a commit. Specifying a glob is equivalent to specifying this
option multiple times, one for each matching ref name.

See also the fetch.negotiationAlgorithm and push.negotiate configuration variables
documented in git-config(1), and the --negotiate-only option below.

--negotiate-only
Do not fetch anything from the server, and instead print the ancestors of the provided
--negotiation-tip=* arguments, which we have in common with the server.

Internally this is used to implement the push.negotiate option, see git-config(1).

--dry-run
Show what would be done, without making any changes.

--[no-]write-fetch-head
Write the list of remote refs fetched in the FETCH_HEAD file directly under $GIT_DIR.
This is the default. Passing --no-write-fetch-head from the command line tells Git not
to write the file. Under --dry-run option, the file is never written.

-f, --force
When git fetch is used with <src>:<dst> refspec it may refuse to update the local
branch as discussed in the <refspec> part below. This option overrides that check.

-k, --keep
Keep downloaded pack.

--multiple
Allow several <repository> and <group> arguments to be specified. No <refspec>s may be
specified.

--[no-]auto-maintenance, --[no-]auto-gc
Run git maintenance run --auto at the end to perform automatic repository maintenance
if needed. (--[no-]auto-gc is a synonym.) This is enabled by default.

--[no-]write-commit-graph
Write a commit-graph after fetching. This overrides the config setting
fetch.writeCommitGraph.

--prefetch
Modify the configured refspec to place all refs into the refs/prefetch/ namespace. See
the prefetch task in git-maintenance(1).

-p, --prune
Before fetching, remove any remote-tracking references that no longer exist on the
remote. Tags are not subject to pruning if they are fetched only because of the
default tag auto-following or due to a --tags option. However, if tags are fetched due
to an explicit refspec (either on the command line or in the remote configuration, for
example if the remote was cloned with the --mirror option), then they are also subject
to pruning. Supplying --prune-tags is a shorthand for providing the tag refspec.

See the PRUNING section below for more details.

-P, --prune-tags
Before fetching, remove any local tags that no longer exist on the remote if --prune
is enabled. This option should be used more carefully, unlike --prune it will remove
any local references (local tags) that have been created. This option is a shorthand
for providing the explicit tag refspec along with --prune, see the discussion about
that in its documentation.

See the PRUNING section below for more details.

-n, --no-tags
By default, tags that point at objects that are downloaded from the remote repository
are fetched and stored locally. This option disables this automatic tag following. The
default behavior for a remote may be specified with the remote.<name>.tagOpt setting.
See git-config(1).

--refmap=<refspec>
When fetching refs listed on the command line, use the specified refspec (can be given
more than once) to map the refs to remote-tracking branches, instead of the values of
remote.*.fetch configuration variables for the remote repository. Providing an empty
<refspec> to the --refmap option causes Git to ignore the configured refspecs and rely
entirely on the refspecs supplied as command-line arguments. See section on
"Configured Remote-tracking Branches" for details.

-t, --tags
Fetch all tags from the remote (i.e., fetch remote tags refs/tags/* into local tags
with the same name), in addition to whatever else would otherwise be fetched. Using
this option alone does not subject tags to pruning, even if --prune is used (though
tags may be pruned anyway if they are also the destination of an explicit refspec; see
--prune).

--recurse-submodules[=yes|on-demand|no]
This option controls if and under what conditions new commits of populated submodules
should be fetched too. It can be used as a boolean option to completely disable
recursion when set to no or to unconditionally recurse into all populated submodules
when set to yes, which is the default when this option is used without any value. Use
on-demand to only recurse into a populated submodule when the superproject retrieves a
commit that updates the submodule's reference to a commit that isn't already in the
local submodule clone. By default, on-demand is used, unless fetch.recurseSubmodules
is set (see git-config(1)).

-j, --jobs=<n>
Number of parallel children to be used for all forms of fetching.

If the --multiple option was specified, the different remotes will be fetched in
parallel. If multiple submodules are fetched, they will be fetched in parallel. To
control them independently, use the config settings fetch.parallel and
submodule.fetchJobs (see git-config(1)).

Typically, parallel recursive and multi-remote fetches will be faster. By default
fetches are performed sequentially, not in parallel.

--no-recurse-submodules
Disable recursive fetching of submodules (this has the same effect as using the
--recurse-submodules=no option).

--set-upstream
If the remote is fetched successfully, add upstream (tracking) reference, used by
argument-less git-pull(1) and other commands. For more information, see
branch.<name>.merge and branch.<name>.remote in git-config(1).

--submodule-prefix=<path>
Prepend <path> to paths printed in informative messages such as "Fetching submodule
foo". This option is used internally when recursing over submodules.

--recurse-submodules-default=[yes|on-demand]
This option is used internally to temporarily provide a non-negative default value for
the --recurse-submodules option. All other methods of configuring fetch's submodule
recursion (such as settings in gitmodules(5) and git-config(1)) override this option,
as does specifying --[no-]recurse-submodules directly.

-u, --update-head-ok
By default git fetch refuses to update the head which corresponds to the current
branch. This flag disables the check. This is purely for the internal use for git pull
to communicate with git fetch, and unless you are implementing your own Porcelain you
are not supposed to use it.

--upload-pack <upload-pack>
When given, and the repository to fetch from is handled by git fetch-pack,
--exec=<upload-pack> is passed to the command to specify non-default path for the
command run on the other end.

-q, --quiet
Pass --quiet to git-fetch-pack and silence any other internally used git commands.
Progress is not reported to the standard error stream.

-v, --verbose
Be verbose.

--progress
Progress status is reported on the standard error stream by default when it is
attached to a terminal, unless -q is specified. This flag forces progress status even
if the standard error stream is not directed to a terminal.

-o <option>, --server-option=<option>
Transmit the given string to the server when communicating using protocol version 2.
The given string must not contain a NUL or LF character. The server's handling of
server options, including unknown ones, is server-specific. When multiple
--server-option=<option> are given, they are all sent to the other side in the order
listed on the command line.

--show-forced-updates
By default, git checks if a branch is force-updated during fetch. This can be disabled
through fetch.showForcedUpdates, but the --show-forced-updates option guarantees this
check occurs. See git-config(1).

--no-show-forced-updates
By default, git checks if a branch is force-updated during fetch. Pass
--no-show-forced-updates or set fetch.showForcedUpdates to false to skip this check
for performance reasons. If used during git-pull the --ff-only option will still check
for forced updates before attempting a fast-forward update. See git-config(1).

-4, --ipv4
Use IPv4 addresses only, ignoring IPv6 addresses.

-6, --ipv6
Use IPv6 addresses only, ignoring IPv4 addresses.

<repository>
The "remote" repository that is the source of a fetch or pull operation. This
parameter can be either a URL (see the section GIT URLS below) or the name of a remote
(see the section REMOTES below).

<group>
A name referring to a list of repositories as the value of remotes.<group> in the
configuration file. (See git-config(1)).

<refspec>
Specifies which refs to fetch and which local refs to update. When no <refspec>s
appear on the command line, the refs to fetch are read from remote.<repository>.fetch
variables instead (see CONFIGURED REMOTE-TRACKING BRANCHES below).

The format of a <refspec> parameter is an optional plus +, followed by the source
<src>, followed by a colon :, followed by the destination ref <dst>. The colon can be
omitted when <dst> is empty. <src> is typically a ref, but it can also be a fully
spelled hex object name.

A <refspec> may contain a * in its <src> to indicate a simple pattern match. Such a
refspec functions like a glob that matches any ref with the same prefix. A pattern
<refspec> must have a * in both the <src> and <dst>. It will map refs to the
destination by replacing the * with the contents matched from the source.

If a refspec is prefixed by ^, it will be interpreted as a negative refspec. Rather
than specifying which refs to fetch or which local refs to update, such a refspec will
instead specify refs to exclude. A ref will be considered to match if it matches at
least one positive refspec, and does not match any negative refspec. Negative refspecs
can be useful to restrict the scope of a pattern refspec so that it will not include
specific refs. Negative refspecs can themselves be pattern refspecs. However, they may
only contain a <src> and do not specify a <dst>. Fully spelled out hex object names
are also not supported.

tag <tag> means the same as refs/tags/<tag>:refs/tags/<tag>; it requests fetching
everything up to the given tag.

The remote ref that matches <src> is fetched, and if <dst> is not an empty string, an
attempt is made to update the local ref that matches it.

Whether that update is allowed without --force depends on the ref namespace it's being
fetched to, the type of object being fetched, and whether the update is considered to
be a fast-forward. Generally, the same rules apply for fetching as when pushing, see
the <refspec>... section of git-push(1) for what those are. Exceptions to those rules
particular to git fetch are noted below.

Until Git version 2.20, and unlike when pushing with git-push(1), any updates to
refs/tags/* would be accepted without + in the refspec (or --force). When fetching, we
promiscuously considered all tag updates from a remote to be forced fetches. Since Git
version 2.20, fetching to update refs/tags/* works the same way as when pushing. I.e.
any updates will be rejected without + in the refspec (or --force).

Unlike when pushing with git-push(1), any updates outside of refs/{tags,heads}/* will
be accepted without + in the refspec (or --force), whether that's swapping e.g. a tree
object for a blob, or a commit for another commit that's doesn't have the previous
commit as an ancestor etc.

Unlike when pushing with git-push(1), there is no configuration which'll amend these
rules, and nothing like a pre-fetch hook analogous to the pre-receive hook.

As with pushing with git-push(1), all of the rules described above about what's not
allowed as an update can be overridden by adding an the optional leading + to a
refspec (or using --force command line option). The only exception to this is that no
amount of forcing will make the refs/heads/* namespace accept a non-commit object.

Note
When the remote branch you want to fetch is known to be rewound and rebased
regularly, it is expected that its new tip will not be descendant of its previous
tip (as stored in your remote-tracking branch the last time you fetched). You
would want to use the + sign to indicate non-fast-forward updates will be needed
for such branches. There is no way to determine or declare that a branch will be
made available in a repository with this behavior; the pulling user simply must
know this is the expected usage pattern for a branch.

--stdin
Read refspecs, one per line, from stdin in addition to those provided as arguments.
The "tag <name>" format is not supported.

GIT URLS
In general, URLs contain information about the transport protocol, the address of the
remote server, and the path to the repository. Depending on the transport protocol, some
of this information may be absent.

Git supports ssh, git, http, and https protocols (in addition, ftp, and ftps can be used
for fetching, but this is inefficient and deprecated; do not use it).

The native transport (i.e. git:// URL) does no authentication and should be used with
caution on unsecured networks.

The following syntaxes may be used with them:

o ssh://[user@]host.xz[:port]/path/to/repo.git/

o git://host.xz[:port]/path/to/repo.git/

o http[s]://host.xz[:port]/path/to/repo.git/

o ftp[s]://host.xz[:port]/path/to/repo.git/

An alternative scp-like syntax may also be used with the ssh protocol:

o [user@]host.xz:path/to/repo.git/

This syntax is only recognized if there are no slashes before the first colon. This helps
differentiate a local path that contains a colon. For example the local path foo:bar could
be specified as an absolute path or ./foo:bar to avoid being misinterpreted as an ssh url.

The ssh and git protocols additionally support ~username expansion:

o ssh://[user@]host.xz[:port]/~[user]/path/to/repo.git/

o git://host.xz[:port]/~[user]/path/to/repo.git/

o [user@]host.xz:/~[user]/path/to/repo.git/

For local repositories, also supported by Git natively, the following syntaxes may be
used:

o /path/to/repo.git/

o file:///path/to/repo.git/

These two syntaxes are mostly equivalent, except when cloning, when the former implies
--local option. See git-clone(1) for details.

git clone, git fetch and git pull, but not git push, will also accept a suitable bundle
file. See git-bundle(1).

When Git doesn't know how to handle a certain transport protocol, it attempts to use the
remote-<transport> remote helper, if one exists. To explicitly request a remote helper,
the following syntax may be used:

o <transport>::<address>

where <address> may be a path, a server and path, or an arbitrary URL-like string
recognized by the specific remote helper being invoked. See gitremote-helpers(7) for
details.

If there are a large number of similarly-named remote repositories and you want to use a
different format for them (such that the URLs you use will be rewritten into URLs that
work), you can create a configuration section of the form:

[url "<actual url base>"]
insteadOf = <other url base>

For example, with this:

[url "git://git.host.xz/"]
insteadOf = host.xz:/path/to/
insteadOf = work:

a URL like "work:repo.git" or like "host.xz:/path/to/repo.git" will be rewritten in any
context that takes a URL to be "git://git.host.xz/repo.git".

If you want to rewrite URLs for push only, you can create a configuration section of the
form:

[url "<actual url base>"]
pushInsteadOf = <other url base>

For example, with this:

[url "ssh://example.org/"]
pushInsteadOf = git://example.org/

a URL like "git://example.org/path/to/repo.git" will be rewritten to
"ssh://example.org/path/to/repo.git" for pushes, but pulls will still use the original
URL.

REMOTES
The name of one of the following can be used instead of a URL as <repository> argument:

o a remote in the Git configuration file: $GIT_DIR/config,

o a file in the $GIT_DIR/remotes directory, or

o a file in the $GIT_DIR/branches directory.

All of these also allow you to omit the refspec from the command line because they each
contain a refspec which git will use by default.

Named remote in configuration file
You can choose to provide the name of a remote which you had previously configured using
git-remote(1), git-config(1) or even by a manual edit to the $GIT_DIR/config file. The URL
of this remote will be used to access the repository. The refspec of this remote will be
used by default when you do not provide a refspec on the command line. The entry in the
config file would appear like this:

[remote "<name>"]
url = <url>
pushurl = <pushurl>
push = <refspec>
fetch = <refspec>

The <pushurl> is used for pushes only. It is optional and defaults to <url>.

Named file in $GIT_DIR/remotes
You can choose to provide the name of a file in $GIT_DIR/remotes. The URL in this file
will be used to access the repository. The refspec in this file will be used as default
when you do not provide a refspec on the command line. This file should have the following
format:

URL: one of the above URL format
Push: <refspec>
Pull: <refspec>

Push: lines are used by git push and Pull: lines are used by git pull and git fetch.
Multiple Push: and Pull: lines may be specified for additional branch mappings.

Named file in $GIT_DIR/branches
You can choose to provide the name of a file in $GIT_DIR/branches. The URL in this file
will be used to access the repository. This file should have the following format:

<url> is required; #<head> is optional.

Depending on the operation, git will use one of the following refspecs, if you don't
provide one on the command line. <branch> is the name of this file in $GIT_DIR/branches
and <head> defaults to master.

git fetch uses:

refs/heads/<head>:refs/heads/<branch>

git push uses:

HEAD:refs/heads/<head>

CONFIGURED REMOTE-TRACKING BRANCHES
You often interact with the same remote repository by regularly and repeatedly fetching
from it. In order to keep track of the progress of such a remote repository, git fetch
allows you to configure remote.<repository>.fetch configuration variables.

Typically such a variable may look like this:

[remote "origin"]
fetch = +refs/heads/*:refs/remotes/origin/*

This configuration is used in two ways:

o When git fetch is run without specifying what branches and/or tags to fetch on the
command line, e.g. git fetch origin or git fetch, remote.<repository>.fetch values
are used as the refspecs--they specify which refs to fetch and which local refs to
update. The example above will fetch all branches that exist in the origin (i.e. any
ref that matches the left-hand side of the value, refs/heads/*) and update the
corresponding remote-tracking branches in the refs/remotes/origin/* hierarchy.

o When git fetch is run with explicit branches and/or tags to fetch on the command line,
e.g. git fetch origin master, the <refspec>s given on the command line determine what
are to be fetched (e.g. master in the example, which is a short-hand for master:,
which in turn means "fetch the master branch but I do not explicitly say what
remote-tracking branch to update with it from the command line"), and the example
command will fetch only the master branch. The remote.<repository>.fetch values
determine which remote-tracking branch, if any, is updated. When used in this way, the
remote.<repository>.fetch values do not have any effect in deciding what gets fetched
(i.e. the values are not used as refspecs when the command-line lists refspecs); they
are only used to decide where the refs that are fetched are stored by acting as a
mapping.

The latter use of the remote.<repository>.fetch values can be overridden by giving the
--refmap=<refspec> parameter(s) on the command line.

PRUNING
Git has a default disposition of keeping data unless it's explicitly thrown away; this
extends to holding onto local references to branches on remotes that have themselves
deleted those branches.

If left to accumulate, these stale references might make performance worse on big and busy
repos that have a lot of branch churn, and e.g. make the output of commands like git
branch -a --contains <commit> needlessly verbose, as well as impacting anything else
that'll work with the complete set of known references.

These remote-tracking references can be deleted as a one-off with either of:

# While fetching
$ git fetch --prune <name>

# Only prune, don't fetch
$ git remote prune <name>

To prune references as part of your normal workflow without needing to remember to run
that, set fetch.prune globally, or remote.<name>.prune per-remote in the config. See git-
config(1).

Here's where things get tricky and more specific. The pruning feature doesn't actually
care about branches, instead it'll prune local <--> remote-references as a function of the
refspec of the remote (see <refspec> and CONFIGURED REMOTE-TRACKING BRANCHES above).

Therefore if the refspec for the remote includes e.g. refs/tags/*:refs/tags/*, or you
manually run e.g. git fetch --prune <name> "refs/tags/*:refs/tags/*" it won't be stale
remote tracking branches that are deleted, but any local tag that doesn't exist on the
remote.

This might not be what you expect, i.e. you want to prune remote <name>, but also
explicitly fetch tags from it, so when you fetch from it you delete all your local tags,
most of which may not have come from the <name> remote in the first place.

So be careful when using this with a refspec like refs/tags/*:refs/tags/*, or any other
refspec which might map references from multiple remotes to the same local namespace.

Since keeping up-to-date with both branches and tags on the remote is a common use-case
the --prune-tags option can be supplied along with --prune to prune local tags that don't
exist on the remote, and force-update those tags that differ. Tag pruning can also be
enabled with fetch.pruneTags or remote.<name>.pruneTags in the config. See git-config(1).

The --prune-tags option is equivalent to having refs/tags/*:refs/tags/* declared in the
refspecs of the remote. This can lead to some seemingly strange interactions:

# These both fetch tags
$ git fetch --no-tags origin 'refs/tags/*:refs/tags/*'
$ git fetch --no-tags --prune-tags origin

The reason it doesn't error out when provided without --prune or its config versions is
for flexibility of the configured versions, and to maintain a 1=1 mapping between what the
command line flags do, and what the configuration versions do.

It's reasonable to e.g. configure fetch.pruneTags=true in ~/.gitconfig to have tags pruned
whenever git fetch --prune is run, without making every invocation of git fetch without
--prune an error.

Pruning tags with --prune-tags also works when fetching a URL instead of a named remote.
These will all prune tags not found on origin:

$ git fetch origin --prune --prune-tags
$ git fetch origin --prune 'refs/tags/*:refs/tags/*'
$ git fetch <url of origin> --prune --prune-tags
$ git fetch <url of origin> --prune 'refs/tags/*:refs/tags/*'

OUTPUT
The output of "git fetch" depends on the transport method used; this section describes the
output when fetching over the Git protocol (either locally or via ssh) and Smart HTTP
protocol.

The status of the fetch is output in tabular form, with each line representing the status
of a single ref. Each line is of the form:

<flag> <summary> <from> -> <to> [<reason>]

The status of up-to-date refs is shown only if the --verbose option is used.

In compact output mode, specified with configuration variable fetch.output, if either
entire <from> or <to> is found in the other string, it will be substituted with * in the
other string. For example, master -> origin/master becomes master -> origin/*.

flag
A single character indicating the status of the ref:

(space)
for a successfully fetched fast-forward;

+
for a successful forced update;

-
for a successfully pruned ref;

t
for a successful tag update;

*
for a successfully fetched new ref;

!
for a ref that was rejected or failed to update; and

=
for a ref that was up to date and did not need fetching.

summary
For a successfully fetched ref, the summary shows the old and new values of the ref in
a form suitable for using as an argument to git log (this is <old>..<new> in most
cases, and <old>...<new> for forced non-fast-forward updates).

from
The name of the remote ref being fetched from, minus its refs/<type>/ prefix. In the
case of deletion, the name of the remote ref is "(none)".

to
The name of the local ref being updated, minus its refs/<type>/ prefix.

reason
A human-readable explanation. In the case of successfully fetched refs, no explanation
is needed. For a failed ref, the reason for failure is described.

EXAMPLES
o Update the remote-tracking branches:

$ git fetch origin

The above command copies all branches from the remote refs/heads/ namespace and stores
them to the local refs/remotes/origin/ namespace, unless the branch.<name>.fetch
option is used to specify a non-default refspec.

o Using refspecs explicitly:

$ git fetch origin +seen:seen maint:tmp

This updates (or creates, as necessary) branches seen and tmp in the local repository
by fetching from the branches (respectively) seen and maint from the remote
repository.

The seen branch will be updated even if it does not fast-forward, because it is
prefixed with a plus sign; tmp will not be.

o Peek at a remote's branch, without configuring the remote in your local repository:

$ git fetch git://git.kernel.org/pub/scm/git/git.git maint
$ git log FETCH_HEAD

The first command fetches the maint branch from the repository at
git://git.kernel.org/pub/scm/git/git.git and the second command uses FETCH_HEAD to
examine the branch with git-log(1). The fetched objects will eventually be removed by
git's built-in housekeeping (see git-gc(1)).

SECURITY
The fetch and push protocols are not designed to prevent one side from stealing data from
the other repository that was not intended to be shared. If you have private data that you
need to protect from a malicious peer, your best option is to store it in another
repository. This applies to both clients and servers. In particular, namespaces on a
server are not effective for read access control; you should only grant read access to a
namespace to clients that you would trust with read access to the entire repository.

The known attack vectors are as follows:

1. The victim sends "have" lines advertising the IDs of objects it has that are not
explicitly intended to be shared but can be used to optimize the transfer if the peer
also has them. The attacker chooses an object ID X to steal and sends a ref to X, but
isn't required to send the content of X because the victim already has it. Now the
victim believes that the attacker has X, and it sends the content of X back to the
attacker later. (This attack is most straightforward for a client to perform on a
server, by creating a ref to X in the namespace the client has access to and then
fetching it. The most likely way for a server to perform it on a client is to "merge"
X into a public branch and hope that the user does additional work on this branch and
pushes it back to the server without noticing the merge.)

2. As in #1, the attacker chooses an object ID X to steal. The victim sends an object Y
that the attacker already has, and the attacker falsely claims to have X and not Y, so
the victim sends Y as a delta against X. The delta reveals regions of X that are
similar to Y to the attacker.

BUGS
Using --recurse-submodules can only fetch new commits in already checked out submodules
right now. When e.g. upstream added a new submodule in the just fetched commits of the
superproject the submodule itself cannot be fetched, making it impossible to check out
that submodule later without having to do a fetch again. This is expected to be fixed in a
future Git version.