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

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

third_party/git/contrib/emacs/README

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+This directory used to contain various modules for Emacs support.
+
+These were added shortly after Git was first released. Since then
+Emacs's own support for Git got better than what was offered by these
+modes. There are also popular 3rd-party Git modes such as Magit which
+offer replacements for these.
+
+The following modules were available, and can be dug up from the Git
+history:
+
+* git.el:
+
+  Wrapper for "git status" that provided access to other git commands.
+
+  Modern alternatives to this include Magit, and VC mode that ships
+  with Emacs.
+
+* git-blame.el:
+
+  A wrapper for "git blame" written before Emacs's own vc-annotate
+  mode learned to invoke git-blame, which can be done via C-x v g.
+
+* vc-git.el:
+
+  This file used to contain the VC-mode backend for git, but it is no
+  longer distributed with git. It is now maintained as part of Emacs
+  and included in standard Emacs distributions starting from version
+  22.2.
+
+  If you have an earlier Emacs version, upgrading to Emacs 22 is
+  recommended, since the VC mode in older Emacs is not generic enough
+  to be able to support git in a reasonable manner, and no attempt has
+  been made to backport vc-git.el.

third_party/git/contrib/emacs/git-blame.el

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+(error "git-blame.el no longer ships with git. It's recommended
+to replace its use with Emacs's own vc-annotate. See
+contrib/emacs/README in git's
+sources (https://github.com/git/git/blob/master/contrib/emacs/README)
+for more info on suggested alternatives and for why this
+happened.")

third_party/git/contrib/emacs/git.el

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+(error "git.el no longer ships with git. It's recommended to
+replace its use with Magit, or simply delete references to git.el
+in your initialization file(s). See contrib/emacs/README in git's
+sources (https://github.com/git/git/blob/master/contrib/emacs/README)
+for suggested alternatives and for why this happened. Emacs's own
+VC mode and Magit are viable alternatives.")