bash completion: difftool accepts the same options as diff

So complete refs, files after the double-dash and some diff options that
make sense for difftool.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>