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

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

third_party/git/Documentation/RelNotes/1.6.4.4.txt

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+GIT v1.6.4.4 Release Notes
+==========================
+
+Fixes since v1.6.4.4
+--------------------
+
+* The workaround for Github server that sometimes gave 500 (Internal server
+  error) response to HEAD requests in 1.6.4.3 introduced a regression that
+  caused re-fetching projects over http to segfault in certain cases due
+  to uninitialized pointer being freed.
+
+* "git pull" on an unborn branch used to consider anything in the work
+  tree and the index discardable.
+
+* "git diff -b/w" did not work well on the incomplete line at the end of
+  the file, due to an incorrect hashing of lines in the low-level xdiff
+  routines.
+
+* "git checkout-index --prefix=$somewhere" used to work when $somewhere is
+  a symbolic link to a directory elsewhere, but v1.6.4.2 broke it.
+
+* "git unpack-objects --strict", invoked when receive.fsckobjects
+  configuration is set in the receiving repository of "git push", did not
+  properly check the objects, especially the submodule links, it received.
+
+Other minor documentation updates are included.