in_merge_bases(): omit unnecessary redundant common ancestor reduction

in_merge_bases(): omit unnecessary redundant common ancestor reduction

The function get_merge_bases() needs to postprocess the result from
merge_bases_many() in order to make sure none of the commit is a
true ancestor of another commit, which is expensive.  However, when
checking if a commit is an ancestor of another commit, we only need
to see if the commit is a common ancestor between the two, and do
not have to care if other common ancestors merge_bases_many() finds
are true merge bases or an ancestor of another merge base.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>