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

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

third_party/git/Documentation/RelNotes/1.7.9.3.txt

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+Git v1.7.9.3 Release Notes
+==========================
+
+Fixes since v1.7.9.2
+--------------------
+
+ * "git p4" (in contrib/) submit the changes to a wrong place when the
+   "--use-client-spec" option is set.
+
+ * The config.mak.autogen generated by optional autoconf support tried
+   to link the binary with -lintl even when libintl.h is missing from
+   the system.
+
+ * When the filter driver exits before reading the content before the
+   main git process writes the contents to be filtered to the pipe to
+   it, the latter could be killed with SIGPIPE instead of ignoring
+   such an event as an error.
+
+ * "git add --refresh <pathspec>" used to warn about unmerged paths
+   outside the given pathspec.
+
+ * The bulk check-in codepath in "git add" streamed contents that
+   needs smudge/clean filters without running them, instead of punting
+   and delegating to the codepath to run filters after slurping
+   everything to core.
+
+ * "git branch --with $that" assumed incorrectly that the user will never
+   ask the question with nonsense value in $that.
+
+ * "git bundle create" produced a corrupt bundle file upon seeing
+   commits with excessively long subject line.
+
+ * When a remote helper exits before reading the blank line from the
+   main git process to signal the end of commands, the latter could be
+   killed with SIGPIPE. Instead we should ignore such event as a
+   non-error.
+
+ * The commit log template given with "git merge --edit" did not have
+   a short instructive text like what "git commit" gives.
+
+ * "git rev-list --verify-objects -q" omitted the extra verification
+   it needs to do over "git rev-list --objects -q" by mistake.
+
+ * "gitweb" used to drop warnings in the log file when "heads" view is
+   accessed in a repository whose HEAD does not point at a valid
+   branch.
+
+ * An invalid regular expression pattern given by an end user made
+   "gitweb" to return garbled response.
+
+Also contains minor fixes and documentation updates.