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>