merge-base: teach "git merge-base" to drive underlying merge_bases_many()

merge-base: teach "git merge-base" to drive underlying merge_bases_many()

Even though the underlying function for get_merge_bases() can compute
a merge base between one existing commit and another (possibly
nonexistent) commit that would be created by merging many commits,
the facility was not available to git-merge-base.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>