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

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

third_party/git/Documentation/RelNotes/1.5.6.1.txt

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+GIT v1.5.6.1 Release Notes
+==========================
+
+Fixes since v1.5.6
+------------------
+
+* Last minute change broke loose object creation on AIX.
+
+* (performance fix) We used to make $GIT_DIR absolute path early in the
+  programs but keeping it relative to the current directory internally
+  gives 1-3 per-cent performance boost.
+
+* bash completion knows the new --graph option to git-log family.
+
+
+* git-diff -c/--cc showed unnecessary "deletion" lines at the context
+  boundary.
+
+* git-for-each-ref ignored %(object) and %(type) requests for tag
+  objects.
+
+* git-merge usage had a typo.
+
+* Rebuilding of git-svn metainfo database did not take rewriteRoot
+  option into account.
+
+* Running "git-rebase --continue/--skip/--abort" before starting a
+  rebase gave nonsense error messages.