Fix merge-base from getting confused.

Fix merge-base from getting confused.

On Sat, 30 Jul 2005, Linus Torvalds wrote:
>
> Yup, it's git-merge-base, and it is confused by the same thing that
> confused git-rev-list.

Hmm.. Here's a tentative fix. I'm not really happy with it, and maybe
somebody else can come up with a better one. I think this one ends up
being quite a bit more expensive than the old one (it will look up _all_
common parents that have a child that isn't common, and then select the
newest one of the bunch), but I haven't really thought it through very
much.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>