diff-highlight: use flush() helper consistently

diff-highlight: use flush() helper consistently

The current flush() helper only shows the queued diff but
does not clear the queue. This is conceptually a bug, but it
works because we only call it once at the end of the
program.

Let's teach it to clear the queue, which will let us use it
in more places (one for now, but more in future patches).

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