Highlight lines of "---" anywhere in email body.

Such lines are intended for use before the diff region begins, as in
the below example::

  Signed-off-by: John Szakmeister <john@example.com>
  Signed-off-by: Junio C Hamano <junio@example.com>
  Reviewed-by: Felipe Contreras <felipe@example.com>
  ---
  I think this fixes most of the stylistic nits.

Therefore, they shouldn't have been setup as an end for a diff region.