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

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

third_party/git/Documentation/RelNotes/2.13.4.txt

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+Git v2.13.4 Release Notes
+=========================
+
+Fixes since v2.13.3
+-------------------
+
+ * Update the character width tables.
+
+ * A recent update broke an alias that contained an uppercase letter,
+   which has been fixed.
+
+ * On Cygwin, similar to Windows, "git push //server/share/repository"
+   ought to mean a repository on a network share that can be accessed
+   locally, but this did not work correctly due to stripping the double
+   slashes at the beginning.
+
+ * The progress meter did not give a useful output when we haven't had
+   0.5 seconds to measure the throughput during the interval.  Instead
+   show the overall throughput rate at the end, which is a much more
+   useful number.
+
+ * We run an early part of "git gc" that deals with refs before
+   daemonising (and not under lock) even when running a background
+   auto-gc, which caused multiple gc processes attempting to run the
+   early part at the same time.  This is now prevented by running the
+   early part also under the GC lock.
+
+Also contains a handful of small code and documentation clean-ups.