combine-diff: show diffstat with the first parent.

combine-diff: show diffstat with the first parent.

Asking for stat (either with --stat or --patch-with-stat) gives
you diffstat for the first parent, even under combine-diff.

While the combined patch is useful to highlight the complexity
and interaction of the parts touched by all branches when
reviewing a merge commit, diffstat is a tool to assess the
extent of damage the merge brings in, and showing stat with the
first parent is more sensible than clever per-parent diffstat.

Signed-off-by: Junio C Hamano <junkio@cox.net>