combine-diff: treat --summary like --stat

combine-diff: treat --summary like --stat

Currently "--cc --summary" on a merge shows nothing. Since we show "--cc
--stat" as a stat against the first parent, and because --summary is
typically used in combination with --stat, it makes sense to treat them
both the same way.

Note that we have to tweak t4013's setup a bit to test this case, as the
existing merges do not have any --summary results against their first
parent. But since the merge at the tip of 'master' does add and remove
files with respect to the second parent, we can just make a reversed
doppelganger merge where the parents are swapped.

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