commit.c: add in_merge_bases_many()

commit.c: add in_merge_bases_many()

Similar to in_merge_bases(commit, other) that returns true when
commit is an ancestor (i.e. in the merge bases between the two) of
the other commit, in_merge_bases_many(commit, n_other, other[])
checks if commit is an ancestor of any of the other[] commits.

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