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

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>