combine-diff: calculate mode_differs earlier

combine-diff: calculate mode_differs earlier

One loop combined both the patch generation and checking
whether there was any mode change to report. Let's factor
that into two separate loops, as we may care about the mode
change even if we are not generating patches (e.g., because
we are showing a binary diff, which will come in a future
patch).

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