rebase-interactive: silence warning when no commits rewritten

If you do a "rebase -i" and don't change any commits,
nothing is rewritten, and we have no REWRITTEN_LIST. The
shell prints out an ugly message:

  $ GIT_EDITOR=true git rebase -i HEAD^
  /path/to/git-rebase--interactive: 1: cannot open
    /path/to/repo/.git/rebase-merge/rewritten-list: No such file
  Successfully rebased and updated refs/heads/master.

We can fix it by not running "notes copy" at all if nothing
was rewritten.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -565,6 +565,7 @@ do_next () {
 			git diff-tree --stat $(cat "$DOTEST"/head)..HEAD
 	} &&
 	{
+		test -s "$REWRITTEN_LIST" &&
 		git notes copy --for-rewrite=rebase < "$REWRITTEN_LIST" ||
 		true # we don't care if this copying failed
 	} &&