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

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

third_party/git/Documentation/RelNotes/1.7.1.2.txt

@@ -0,0 +1,28 @@
+Git v1.7.1.2 Release Notes
+==========================
+
+Fixes since v1.7.1.1
+--------------------
+
+ * "git commit" did not honor GIT_REFLOG_ACTION environment variable, resulting
+   reflog messages for cherry-pick and revert actions to be recorded as "commit".
+
+ * "git clone/fetch/pull" issued an incorrect error message when a ref and
+   a symref that points to the ref were updated at the same time.  This
+   obviously would update them to the same value, and should not result in
+   an error condition.
+
+ * "git diff" inside a tree with many pathnames that have certain
+   characters has become very slow in 1.7.0 by mistake.
+
+ * "git rev-parse --parseopt --stop-at-non-option" did not stop at non option
+   when --keep-dashdash was in effect.
+
+ * An overlong line after ".gitdir: " in a git file caused out of bounds
+   access to an array on the stack.
+
+ * "git config --path conf.var" to attempt to expand a variable conf.var
+   that uses "~/" short-hand segfaulted when $HOME environment variable
+   was not set.
+
+And other minor fixes and documentation updates.