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>

combine-diff.c

@@ -1326,6 +1326,7 @@ static const char *path_path(void *obj)
  */
 #define STAT_FORMAT_MASK (DIFF_FORMAT_NUMSTAT \
 			  | DIFF_FORMAT_SHORTSTAT \
+			  | DIFF_FORMAT_SUMMARY \
 			  | DIFF_FORMAT_DIFFSTAT)
 
 /* find set of paths that every parent touches */