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

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

third_party/git/Documentation/RelNotes/2.13.2.txt

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+Git v2.13.2 Release Notes
+=========================
+
+Fixes since v2.13.1
+-------------------
+
+ * The "collision detecting" SHA-1 implementation shipped with 2.13.1
+   was still broken on some platforms.  Update to the upstream code
+   again to take their fix.
+
+ * "git checkout --recurse-submodules" did not quite work with a
+   submodule that itself has submodules.
+
+ * Introduce the BUG() macro to improve die("BUG: ...").
+
+ * The "run-command" API implementation has been made more robust
+   against dead-locking in a threaded environment.
+
+ * A recent update to t5545-push-options.sh started skipping all the
+   tests in the script when a web server testing is disabled or
+   unavailable, not just the ones that require a web server.  Non HTTP
+   tests have been salvaged to always run in this script.
+
+ * "git clean -d" used to clean directories that has ignored files,
+   even though the command should not lose ignored ones without "-x".
+   "git status --ignored"  did not list ignored and untracked files
+   without "-uall".  These have been corrected.
+
+ * The timestamp of the index file is now taken after the file is
+   closed, to help Windows, on which a stale timestamp is reported by
+   fstat() on a file that is opened for writing and data was written
+   but not yet closed.
+
+ * "git pull --rebase --autostash" didn't auto-stash when the local history
+   fast-forwards to the upstream.
+
+ * "git describe --contains" penalized light-weight tags so much that
+   they were almost never considered.  Instead, give them about the
+   same chance to be considered as an annotated tag that is the same
+   age as the underlying commit would.
+
+ * The result from "git diff" that compares two blobs, e.g. "git diff
+   $commit1:$path $commit2:$path", used to be shown with the full
+   object name as given on the command line, but it is more natural to
+   use the $path in the output and use it to look up .gitattributes.
+
+ * A flaky test has been corrected.
+
+ * Help contributors that visit us at GitHub.
+
+ * "git stash push <pathspec>" did not work from a subdirectory at all.
+   Bugfix for a topic in v2.13
+
+Also contains various documentation updates and code clean-ups.