git-diff A...B to (usually) mean "git-diff `git-merge-base A B` B"

git-diff A...B to (usually) mean "git-diff `git-merge-base A B` B"

This tweaks the argument parser of "git diff" to allow "git-diff
A...B" to show diffs leading to B since their merge-base, when
there is only one sensible merge base between A and B.

Currently nonsense cases are thrown at combined-diff to produce
nonsense results, which would eventually need to be fixed.

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