diff A...B: give one possible diff when there are more than one merge-base

diff A...B: give one possible diff when there are more than one merge-base

We instead showed a combined diff that explains one of the randomly
chosen merge-base as if it were the result of merging all the other
merge bases and two tips given, which made no sense at all.

An alternative is to simply fail such a request, telling the user that
there are criss-cross merges, but it wouldn't be so helpful.

Noticed by James Pickens.

Signed-off-by: Junio C Hamano <gitster@pobox.com>