combine-diff: treat --dirstat like --stat

combine-diff: treat --dirstat like --stat

Currently "--cc --dirstat" will show nothing for a merge.  Like
--shortstat and --summary in the previous two patches, it probably makes
sense to treat it like we do --stat, and show a stat against the
first-parent.

This case is less obviously correct than for --shortstat and --summary,
as those are basically variants of --stat themselves. It's possible we
could develop a multi-parent combined dirstat format, in which case we
might regret defining this first-parent behavior. But the same could be
said for --stat, and in the 12+ years of it showing first-parent stats,
nobody has complained.

So showing the first-parent dirstat is at least _useful_, and if we
later develop a clever multi-parent stat format, we'd probably have to
deal with --stat anyway.

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