rebase: use plumbing to show dirty state

rebase: use plumbing to show dirty state

Commit 4cfbe06 introduced the use of "git diff" to show
dirty state in a format more familiar to users. However, it
should have used the plumbing "git diff-files" instead.

Not only is it good practice in general to use plumbing in
scripts, but in this case we really don't want the automatic
pager to kick in for an error message.

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