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

Merge commit '1b593e1ea4' as 'third_party/git'

third_party/git/contrib/hooks/multimail/CONTRIBUTING.rst

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+Contributing
+============
+
+git-multimail is an open-source project, built by volunteers. We would
+welcome your help!
+
+The current maintainers are `Matthieu Moy <http://matthieu-moy.fr>`__ and
+`Michael Haggerty <https://github.com/mhagger>`__.
+
+Please note that although a copy of git-multimail is distributed in
+the "contrib" section of the main Git project, development takes place
+in a separate `git-multimail repository on GitHub`_.
+
+Whenever enough changes to git-multimail have accumulated, a new
+code-drop of git-multimail will be submitted for inclusion in the Git
+project.
+
+We use the GitHub issue tracker to keep track of bugs and feature
+requests, and we use GitHub pull requests to exchange patches (though,
+if you prefer, you can send patches via the Git mailing list with CC
+to the maintainers). Please sign off your patches as per the `Git
+project practice
+<https://github.com/git/git/blob/master/Documentation/SubmittingPatches#L234>`__.
+
+Please vote for issues you would like to be addressed in priority
+(click "add your reaction" and then the "+1" thumbs-up button on the
+GitHub issue).
+
+General discussion of git-multimail can take place on the main `Git
+mailing list`_.
+
+Please CC emails regarding git-multimail to the maintainers so that we
+don't overlook them.
+
+Help needed: testers/maintainer for specific environments/OS
+------------------------------------------------------------
+
+The current maintainer uses and tests git-multimail on Linux with the
+Generic environment. More testers, or better contributors are needed
+to test git-multimail on other real-life setups:
+
+* Mac OS X, Windows: git-multimail is currently not supported on these
+  platforms. But since we have no external dependencies and try to
+  write code as portable as possible, it is possible that
+  git-multimail already runs there and if not, it is likely that it
+  could be ported easily.
+
+  Patches to improve support for Windows and OS X are welcome.
+  Ideally, there would be a sub-maintainer for each OS who would test
+  at least once before each release (around twice a year).
+
+* Gerrit, Stash, Gitolite environments: although the testsuite
+  contains tests for these environments, a tester/maintainer for each
+  environment would be welcome to test and report failure (or success)
+  on real-life environments periodically (here also, feedback before
+  each release would be highly appreciated).
+
+
+.. _`git-multimail repository on GitHub`: https://github.com/git-multimail/git-multimail
+.. _`Git mailing list`: git@vger.kernel.org