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

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

third_party/git/Documentation/RelNotes/1.6.5.5.txt

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+Git v1.6.5.5 Release Notes
+==========================
+
+Fixes since v1.6.5.4
+--------------------
+
+ * Manual pages can be formatted with older xmlto again.
+
+ * GREP_OPTIONS exported from user's environment could have broken
+   our scripted commands.
+
+ * In configuration files, a few variables that name paths can begin with
+   ~/ and ~username/ and they are expanded as expected.  This is not a
+   bugfix but 1.6.6 will have this and without backporting users cannot
+   easily use the same ~/.gitconfig across versions.
+
+ * "git diff -B -M" did the same computation to hash lines of contents
+   twice, and held onto memory after it has used the data in it
+   unnecessarily before it freed.
+
+ * "git diff -B" and "git diff --dirstat" was not counting newly added
+   contents correctly.
+
+ * "git format-patch revisions... -- path" issued an incorrect error
+   message that suggested to use "--" on the command line when path
+   does not exist in the current work tree (it is a separate matter if
+   it makes sense to limit format-patch with pathspecs like that
+   without using the --full-diff option).
+
+ * "git grep -F -i StRiNg" did not work as expected.
+
+ * Enumeration of available merge strategies iterated over the list of
+   commands in a wrong way, sometimes producing an incorrect result.
+
+ * "git shortlog" did not honor the "encoding" header embedded in the
+   commit object like "git log" did.
+
+ * Reading progress messages that come from the remote side while running
+   "git pull" is given precedence over reading the actual pack data to
+   prevent garbled progress message on the user's terminal.
+
+ * "git rebase" got confused when the log message began with certain
+   strings that looked like Subject:, Date: or From: header.
+
+ * "git reset" accidentally run in .git/ directory checked out the
+   work tree contents in there.
+
+
+Other minor documentation updates are included.