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

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

third_party/git/Documentation/RelNotes/1.7.6.3.txt

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+Git v1.7.6.3 Release Notes
+==========================
+
+Fixes since v1.7.6.2
+--------------------
+
+ * "git -c var=value subcmd" misparsed the custom configuration when
+   value contained an equal sign.
+
+ * "git fetch" had a major performance regression, wasting many
+   needless cycles in a repository where there is no submodules
+   present. This was especially bad, when there were many refs.
+
+ * "git reflog $refname" did not default to the "show" subcommand as
+   the documentation advertised the command to do.
+
+ * "git reset" did not leave meaningful log message in the reflog.
+
+ * "git status --ignored" did not show ignored items when there is no
+   untracked items.
+
+ * "git tag --contains $commit" was unnecessarily inefficient.
+
+Also contains minor fixes and documentation updates.