merge(third_party/git): Merge squashed git subtree at v2.23.0 r/373

Merge commit '1b593e1ea4d2af0f6444d9a7788d5d99abd6fde5' as 'third_party/git'

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diff --git a/third_party/git/contrib/subtree/git-subtree.txt b/third_party/git/contrib/subtree/git-subtree.txt
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+git-subtree(1)
+==============
+
+NAME
+----
+git-subtree - Merge subtrees together and split repository into subtrees
+
+
+SYNOPSIS
+--------
+[verse]
+'git subtree' add   -P <prefix> <commit>
+'git subtree' add   -P <prefix> <repository> <ref>
+'git subtree' pull  -P <prefix> <repository> <ref>
+'git subtree' push  -P <prefix> <repository> <ref>
+'git subtree' merge -P <prefix> <commit>
+'git subtree' split -P <prefix> [OPTIONS] [<commit>]
+
+
+DESCRIPTION
+-----------
+Subtrees allow subprojects to be included within a subdirectory
+of the main project, optionally including the subproject's
+entire history.
+
+For example, you could include the source code for a library
+as a subdirectory of your application.
+
+Subtrees are not to be confused with submodules, which are meant for
+the same task. Unlike submodules, subtrees do not need any special
+constructions (like .gitmodules files or gitlinks) be present in
+your repository, and do not force end-users of your
+repository to do anything special or to understand how subtrees
+work. A subtree is just a subdirectory that can be
+committed to, branched, and merged along with your project in
+any way you want.
+
+They are also not to be confused with using the subtree merge
+strategy. The main difference is that, besides merging
+the other project as a subdirectory, you can also extract the
+entire history of a subdirectory from your project and make it
+into a standalone project. Unlike the subtree merge strategy
+you can alternate back and forth between these
+two operations. If the standalone library gets updated, you can
+automatically merge the changes into your project; if you
+update the library inside your project, you can "split" the
+changes back out again and merge them back into the library
+project.
+
+For example, if a library you made for one application ends up being
+useful elsewhere, you can extract its entire history and publish
+that as its own git repository, without accidentally
+intermingling the history of your application project.
+
+[TIP]
+In order to keep your commit messages clean, we recommend that
+people split their commits between the subtrees and the main
+project as much as possible.  That is, if you make a change that
+affects both the library and the main application, commit it in
+two pieces.  That way, when you split the library commits out
+later, their descriptions will still make sense.  But if this
+isn't important to you, it's not *necessary*.  git subtree will
+simply leave out the non-library-related parts of the commit
+when it splits it out into the subproject later.
+
+
+COMMANDS
+--------
+add::
+	Create the <prefix> subtree by importing its contents
+	from the given <commit> or <repository> and remote <ref>.
+	A new commit is created	automatically, joining the imported
+	project's history with your own.  With '--squash', imports
+	only a single commit from the subproject, rather than its
+	entire history.
+
+merge::
+	Merge recent changes up to <commit> into the <prefix>
+	subtree.  As with normal 'git merge', this doesn't
+	remove your own local changes; it just merges those
+	changes into the latest <commit>.  With '--squash',
+	creates only one commit that contains all the changes,
+	rather than merging in the entire history.
++
+If you use '--squash', the merge direction doesn't always have to be
+forward; you can use this command to go back in time from v2.5 to v2.4,
+for example.  If your merge introduces a conflict, you can resolve it in
+the usual ways.
+	
+pull::
+	Exactly like 'merge', but parallels 'git pull' in that
+	it fetches the given ref from the specified remote
+	repository.
+	
+push::
+	Does a 'split' (see below) using the <prefix> supplied
+	and then does a 'git push' to push the result to the 
+	repository and ref. This can be used to push your
+	subtree to different branches of the remote repository.
+
+split::
+	Extract a new, synthetic project history from the
+	history of the <prefix> subtree.  The new history
+	includes only the commits (including merges) that
+	affected <prefix>, and each of those commits now has the
+	contents of <prefix> at the root of the project instead
+	of in a subdirectory.  Thus, the newly created history
+	is suitable for export as a separate git repository.
++
+After splitting successfully, a single commit id is printed to stdout.
+This corresponds to the HEAD of the newly created tree, which you can
+manipulate however you want.
++
+Repeated splits of exactly the same history are guaranteed to be
+identical (i.e. to produce the same commit ids).  Because of this, if
+you add new commits and then re-split, the new commits will be attached
+as commits on top of the history you generated last time, so 'git merge'
+and friends will work as expected.
++
+Note that if you use '--squash' when you merge, you should usually not
+just '--rejoin' when you split.
+
+
+OPTIONS
+-------
+-q::
+--quiet::
+	Suppress unnecessary output messages on stderr.
+
+-d::
+--debug::
+	Produce even more unnecessary output messages on stderr.
+
+-P <prefix>::
+--prefix=<prefix>::
+	Specify the path in the repository to the subtree you
+	want to manipulate.  This option is mandatory
+	for all commands.
+
+-m <message>::
+--message=<message>::
+	This option is only valid for add, merge and pull (unsure).
+	Specify <message> as the commit message for the merge commit.
+
+
+OPTIONS FOR add, merge, push, pull
+----------------------------------
+--squash::
+	This option is only valid for add, merge, and pull
+	commands.
++
+Instead of merging the entire history from the subtree project, produce
+only a single commit that contains all the differences you want to
+merge, and then merge that new commit into your project.
++
+Using this option helps to reduce log clutter. People rarely want to see
+every change that happened between v1.0 and v1.1 of the library they're
+using, since none of the interim versions were ever included in their
+application.
++
+Using '--squash' also helps avoid problems when the same subproject is
+included multiple times in the same project, or is removed and then
+re-added.  In such a case, it doesn't make sense to combine the
+histories anyway, since it's unclear which part of the history belongs
+to which subtree.
++
+Furthermore, with '--squash', you can switch back and forth between
+different versions of a subtree, rather than strictly forward.  'git
+subtree merge --squash' always adjusts the subtree to match the exactly
+specified commit, even if getting to that commit would require undoing
+some changes that were added earlier.
++
+Whether or not you use '--squash', changes made in your local repository
+remain intact and can be later split and send upstream to the
+subproject.
+
+
+OPTIONS FOR split
+-----------------
+--annotate=<annotation>::
+	This option is only valid for the split command.
++
+When generating synthetic history, add <annotation> as a prefix to each
+commit message.  Since we're creating new commits with the same commit
+message, but possibly different content, from the original commits, this
+can help to differentiate them and avoid confusion.
++
+Whenever you split, you need to use the same <annotation>, or else you
+don't have a guarantee that the new re-created history will be identical
+to the old one.  That will prevent merging from working correctly.  git
+subtree tries to make it work anyway, particularly if you use --rejoin,
+but it may not always be effective.
+
+-b <branch>::
+--branch=<branch>::
+	This option is only valid for the split command.
++
+After generating the synthetic history, create a new branch called
+<branch> that contains the new history.  This is suitable for immediate
+pushing upstream.  <branch> must not already exist.
+
+--ignore-joins::
+	This option is only valid for the split command.
++
+If you use '--rejoin', git subtree attempts to optimize its history
+reconstruction to generate only the new commits since the last
+'--rejoin'.  '--ignore-join' disables this behaviour, forcing it to
+regenerate the entire history.  In a large project, this can take a long
+time.
+
+--onto=<onto>::
+	This option is only valid for the split command.
++
+If your subtree was originally imported using something other than git
+subtree, its history may not match what git subtree is expecting.  In
+that case, you can specify the commit id <onto> that corresponds to the
+first revision of the subproject's history that was imported into your
+project, and git subtree will attempt to build its history from there.
++
+If you used 'git subtree add', you should never need this option.
+
+--rejoin::
+	This option is only valid for the split command.
++
+After splitting, merge the newly created synthetic history back into
+your main project.  That way, future splits can search only the part of
+history that has been added since the most recent --rejoin.
++
+If your split commits end up merged into the upstream subproject, and
+then you want to get the latest upstream version, this will allow git's
+merge algorithm to more intelligently avoid conflicts (since it knows
+these synthetic commits are already part of the upstream repository).
++
+Unfortunately, using this option results in 'git log' showing an extra
+copy of every new commit that was created (the original, and the
+synthetic one).
++
+If you do all your merges with '--squash', don't use '--rejoin' when you
+split, because you don't want the subproject's history to be part of
+your project anyway.
+
+
+EXAMPLE 1. Add command
+----------------------
+Let's assume that you have a local repository that you would like
+to add an external vendor library to. In this case we will add the
+git-subtree repository as a subdirectory of your already existing
+git-extensions repository in ~/git-extensions/:
+
+	$ git subtree add --prefix=git-subtree --squash \
+		git://github.com/apenwarr/git-subtree.git master
+
+'master' needs to be a valid remote ref and can be a different branch
+name
+
+You can omit the --squash flag, but doing so will increase the number
+of commits that are included in your local repository.
+
+We now have a ~/git-extensions/git-subtree directory containing code
+from the master branch of git://github.com/apenwarr/git-subtree.git
+in our git-extensions repository.
+
+EXAMPLE 2. Extract a subtree using commit, merge and pull
+---------------------------------------------------------
+Let's use the repository for the git source code as an example.
+First, get your own copy of the git.git repository:
+
+	$ git clone git://git.kernel.org/pub/scm/git/git.git test-git
+	$ cd test-git
+
+gitweb (commit 1130ef3) was merged into git as of commit
+0a8f4f0, after which it was no longer maintained separately. 
+But imagine it had been maintained separately, and we wanted to
+extract git's changes to gitweb since that time, to share with
+the upstream.  You could do this:
+
+	$ git subtree split --prefix=gitweb --annotate='(split) ' \
+        	0a8f4f0^.. --onto=1130ef3 --rejoin \
+        	--branch gitweb-latest
+        $ gitk gitweb-latest
+        $ git push git@github.com:whatever/gitweb.git gitweb-latest:master
+        
+(We use '0a8f4f0^..' because that means "all the changes from
+0a8f4f0 to the current version, including 0a8f4f0 itself.")
+
+If gitweb had originally been merged using 'git subtree add' (or
+a previous split had already been done with --rejoin specified)
+then you can do all your splits without having to remember any
+weird commit ids:
+
+	$ git subtree split --prefix=gitweb --annotate='(split) ' --rejoin \
+		--branch gitweb-latest2
+
+And you can merge changes back in from the upstream project just
+as easily:
+
+	$ git subtree pull --prefix=gitweb \
+		git@github.com:whatever/gitweb.git master
+
+Or, using '--squash', you can actually rewind to an earlier
+version of gitweb:
+
+	$ git subtree merge --prefix=gitweb --squash gitweb-latest~10
+
+Then make some changes:
+
+	$ date >gitweb/myfile
+	$ git add gitweb/myfile
+	$ git commit -m 'created myfile'
+
+And fast forward again:
+
+	$ git subtree merge --prefix=gitweb --squash gitweb-latest
+
+And notice that your change is still intact:
+	
+	$ ls -l gitweb/myfile
+
+And you can split it out and look at your changes versus
+the standard gitweb:
+
+	git log gitweb-latest..$(git subtree split --prefix=gitweb)
+
+EXAMPLE 3. Extract a subtree using branch
+-----------------------------------------
+Suppose you have a source directory with many files and
+subdirectories, and you want to extract the lib directory to its own
+git project. Here's a short way to do it:
+
+First, make the new repository wherever you want:
+
+	$ <go to the new location>
+	$ git init --bare
+
+Back in your original directory:
+
+	$ git subtree split --prefix=lib --annotate="(split)" -b split
+
+Then push the new branch onto the new empty repository:
+
+	$ git push <new-repo> split:master
+
+
+AUTHOR
+------
+Written by Avery Pennarun <apenwarr@gmail.com>
+
+
+GIT
+---
+Part of the linkgit:git[1] suite