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

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

third_party/git/Documentation/RelNotes/2.4.6.txt

@@ -0,0 +1,23 @@
+Git v2.4.6 Release Notes
+========================
+
+Fixes since v2.4.5
+------------------
+
+ * "git fetch --depth=<depth>" and "git clone --depth=<depth>" issued
+   a shallow transfer request even to an upload-pack that does not
+   support the capability.
+
+ * "git fsck" used to ignore missing or invalid objects recorded in reflog.
+
+ * The tcsh completion writes a bash scriptlet but that would have
+   failed for users with noclobber set.
+
+ * Recent Mac OS X updates breaks the logic to detect that the machine
+   is on the AC power in the sample pre-auto-gc script.
+
+ * "git format-patch --ignore-if-upstream A..B" did not like to be fed
+   tags as boundary commits.
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.