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

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

third_party/git/Documentation/RelNotes/1.7.9.4.txt

@@ -0,0 +1,24 @@
+Git v1.7.9.4 Release Notes
+==========================
+
+Fixes since v1.7.9.3
+--------------------
+
+ * The code to synthesize the fake ancestor tree used by 3-way merge
+   fallback in "git am" was not prepared to read a patch created with
+   a non-standard -p<num> value.
+
+ * "git bundle" did not record boundary commits correctly when there
+   are many of them.
+
+ * "git diff-index" and its friends at the plumbing level showed the
+   "diff --git" header and nothing else for a path whose cached stat
+   info is dirty without actual difference when asked to produce a
+   patch. This was a longstanding bug that we could have fixed long
+   time ago.
+
+ * "gitweb" did use quotemeta() to prepare search string when asked to
+   do a fixed-string project search, but did not use it by mistake and
+   used the user-supplied string instead.
+
+Also contains minor fixes and documentation updates.