combine-diff: treat --shortstat like --stat

The --stat of a combined diff is defined as the first-parent stat,
going all the way back to 965f803c32 (combine-diff: show diffstat with
the first parent., 2006-04-17).

Naturally, we gave --numstat the same treatment in 74e2abe5b7 (diff
--numstat, 2006-10-12).

But --shortstat, which is really just the final line of --stat, does
nothing, which produces confusing results:

  $ git show --oneline --stat eab7584e37
  eab7584e37 Merge branch 'en/show-ref-doc-fix'

   Documentation/git-show-ref.txt | 2 +-
   1 file changed, 1 insertion(+), 1 deletion(-)

  $ git show --oneline --shortstat eab7584e37
  eab7584e37 Merge branch 'en/show-ref-doc-fix'

  [nothing! We'd expect to see the "1 file changed..." line]

This patch teaches combine-diff to treats the two formats identically.

Reported-by: David Turner <novalis@novalis.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

combine-diff.c

@@ -1325,6 +1325,7 @@ static const char *path_path(void *obj)
  * Diff stat formats which we always compute solely against the first parent.
  */
 #define STAT_FORMAT_MASK (DIFF_FORMAT_NUMSTAT \
+			  | DIFF_FORMAT_SHORTSTAT \
 			  | DIFF_FORMAT_DIFFSTAT)
 
 /* find set of paths that every parent touches */

t/t4013-diff-various.sh

@@ -239,6 +239,7 @@ diff-tree --cc --stat --summary master
 # stat summary should show the diffstat and summary with the first parent
 diff-tree -c --stat --summary side
 diff-tree --cc --stat --summary side
+diff-tree --cc --shortstat master
 # improved by Timo's patch
 diff-tree --cc --patch-with-stat master
 # improved by Timo's patch

t/t4013/diff.diff-tree_--cc_--shortstat_master

@@ -0,0 +1,4 @@
+$ git diff-tree --cc --shortstat master
+59d314ad6f356dd08601a4cd5e530381da3e3c64
+ 2 files changed, 5 insertions(+)
+$