Allow in_merge_bases() to take more than one reference commits.

Allow in_merge_bases() to take more than one reference commits.

The internal function in_merge_bases(A, B) is used to make sure
that commit A is an ancestor of commit B.  This changes the
signature of it to take an array of B's and updates its current
callers.

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