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

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

third_party/git/Documentation/RelNotes/1.8.1.6.txt

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+Git 1.8.1.6 Release Notes
+=========================
+
+Fixes since v1.8.1.5
+--------------------
+
+ * An earlier change to the attribute system introduced at v1.8.1.2 by
+   mistake stopped a pattern "dir" (without trailing slash) from
+   matching a directory "dir" (it only wanted to allow pattern "dir/"
+   to also match).
+
+ * The code to keep track of what directory names are known to Git on
+   platforms with case insensitive filesystems can get confused upon a
+   hash collision between these pathnames and looped forever.
+
+ * When the "--prefix" option is used to "checkout-index", the code
+   did not pick the correct output filter based on the attribute
+   setting.
+
+ * Annotated tags outside refs/tags/ hierarchy were not advertised
+   correctly to the ls-remote and fetch with recent version of Git.
+
+ * The logic used by "git diff -M --stat" to shorten the names of
+   files before and after a rename did not work correctly when the
+   common prefix and suffix between the two filenames overlapped.
+
+ * "git update-index -h" did not do the usual "-h(elp)" thing.
+
+ * perl/Git.pm::cat_blob slurped everything in core only to write it
+   out to a file descriptor, which was not a very smart thing to do.
+
+ * The SSL peer verification done by "git imap-send" did not ask for
+   Server Name Indication (RFC 4366), failing to connect SSL/TLS
+   sites that serve multiple hostnames on a single IP.
+
+ * "git bundle verify" did not say "records a complete history" for a
+   bundle that does not have any prerequisites.
+
+Also contains various documentation fixes.