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>

t/t4013-diff-various.sh

@@ -98,6 +98,12 @@ test_expect_success setup '
 	git commit -m "update mode" &&
 	git checkout -f master &&
 
+	# Same merge as master, but with parents reversed. Hide it in a
+	# pseudo-ref to avoid impacting tests with --all.
+	commit=$(echo reverse |
+		 git commit-tree -p master^2 -p master^1 master^{tree}) &&
+	git update-ref REVERSE $commit &&
+
 	git config diff.renames false &&
 
 	git show-branch
@@ -240,6 +246,7 @@ diff-tree --cc --stat --summary master
 diff-tree -c --stat --summary side
 diff-tree --cc --stat --summary side
 diff-tree --cc --shortstat master
+diff-tree --cc --summary REVERSE
 # improved by Timo's patch
 diff-tree --cc --patch-with-stat master
 # improved by Timo's patch