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

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

third_party/git/Documentation/RelNotes/1.7.5.1.txt

@@ -0,0 +1,47 @@
+Git v1.7.5.1 Release Notes
+==========================
+
+Fixes since v1.7.5
+------------------
+
+ * When an object "$tree:$path" does not exist, if $path does exist in the
+   subtree of $tree that corresponds to the subdirectory the user is in,
+   git now suggests using "$tree:./$path" in addition to the advice to use
+   the full path from the root of the working tree.
+
+ * The "--date=relative" output format used to say "X years, 12 months"
+   when it should have said "X+1 years".
+
+ * The smart-HTTP transfer was broken in 1.7.5 when the client needs
+   to issue a small POST (which uses content-length) and then a large
+   POST (which uses chunked) back to back.
+
+ * "git clean" used to fail on an empty directory that is not readable,
+   even though rmdir(2) could remove such a directory.  Now we attempt it
+   as the last resort.
+
+ * The "--dirstat" option of "diff" family of commands used to totally
+   ignore a change that only rearranged lines within a file.  Such a
+   change now counts as at least a minimum but non zero change.
+
+ * The "--dirstat" option of "diff" family of commands used to use the
+   pathname in the original, instead of the pathname in the result,
+   when renames are involved.
+
+ * "git pack-object" did not take core.bigfilethreashold into account
+   (unlike fast-import); now it does.
+
+ * "git reflog" ignored options like "--format=.." on the command line.
+
+ * "git stash apply" used to refuse to work if there was any change in
+   the working tree, even when the change did not overlap with the change
+   the stash recorded.
+
+ * "git stash apply @{99999}" was not diagnosed as an error, even when you
+   did not have that many stash entries.
+
+ * An error message from "git send-email" to diagnose a broken SMTP
+   connection configuration lacked a space between "hello=<smtp-domain>"
+   and "port=<smtp-server-port>".
+
+And other minor fixes and documentation updates.