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

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

third_party/git/Documentation/RelNotes/1.6.0.1.txt

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+GIT v1.6.0.1 Release Notes
+==========================
+
+Fixes since v1.6.0
+------------------
+
+* "git diff --cc" did not honor content mangling specified by
+  gitattributes and core.autocrlf when reading from the work tree.
+
+* "git diff --check" incorrectly detected new trailing blank lines when
+  whitespace check was in effect.
+
+* "git for-each-ref" tried to dereference NULL when asked for '%(body)" on
+  a tag with a single incomplete line as its payload.
+
+* "git format-patch" peeked before the beginning of a string when
+  "format.headers" variable is empty (a misconfiguration).
+
+* "git help help" did not work correctly.
+
+* "git mailinfo" (hence "git am") was unhappy when MIME multipart message
+  contained garbage after the finishing boundary.
+
+* "git mailinfo" also was unhappy when the "From: " line only had a bare
+  e-mail address.
+
+* "git merge" did not refresh the index correctly when a merge resulted in
+  a fast-forward.
+
+* "git merge" did not resolve a truly trivial merges that can be done
+  without content level merges.
+
+* "git svn dcommit" to a repository with URL that has embedded usernames
+  did not work correctly.
+
+Contains other various documentation fixes.