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

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

third_party/git/Documentation/RelNotes/2.6.5.txt

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+Git v2.6.5 Release Notes
+========================
+
+Fixes since v2.6.4
+------------------
+
+ * Because "test_when_finished" in our test framework queues the
+   clean-up tasks to be done in a shell variable, it should not be
+   used inside a subshell.  Add a mechanism to allow 'bash' to catch
+   such uses, and fix the ones that were found.
+
+ * Update "git subtree" (in contrib/) so that it can take whitespaces
+   in the pathnames, not only in the in-tree pathname but the name of
+   the directory that the repository is in.
+
+ * Cosmetic improvement to lock-file error messages.
+
+ * mark_tree_uninteresting() has code to handle the case where it gets
+   passed a NULL pointer in its 'tree' parameter, but the function had
+   'object = &tree->object' assignment before checking if tree is
+   NULL.  This gives a compiler an excuse to declare that tree will
+   never be NULL and apply a wrong optimization.  Avoid it.
+
+ * The helper used to iterate over loose object directories to prune
+   stale objects did not closedir() immediately when it is done with a
+   directory--a callback such as the one used for "git prune" may want
+   to do rmdir(), but it would fail on open directory on platforms
+   such as WinXP.
+
+ * "git p4" used to import Perforce CLs that touch only paths outside
+   the client spec as empty commits.  It has been corrected to ignore
+   them instead, with a new configuration git-p4.keepEmptyCommits as a
+   backward compatibility knob.
+
+ * The exit code of git-fsck did not reflect some types of errors
+   found in packed objects, which has been corrected.
+
+ * The completion script (in contrib/) used to list "git column"
+   (which is not an end-user facing command) as one of the choices
+
+ * Improve error reporting when SMTP TLS fails.
+
+ * When getpwuid() on the system returned NULL (e.g. the user is not
+   in the /etc/passwd file or other uid-to-name mappings), the
+   codepath to find who the user is to record it in the reflog barfed
+   and died.  Loosen the check in this codepath, which already accepts
+   questionable ident string (e.g. host part of the e-mail address is
+   obviously bogus), and in general when we operate fmt_ident() function
+   in non-strict mode.
+
+ * "git symbolic-ref" forgot to report a failure with its exit status.
+
+ * History traversal with "git log --source" that starts with an
+   annotated tag failed to report the tag as "source", due to an
+   old regression in the command line parser back in v2.2 days.
+
+Also contains typofixes, documentation updates and trivial code
+clean-ups.