Introduce get_merge_bases_many()

This introduces a new function get_merge_bases_many() which is a natural
extension of two commit merge base computation.  It is given one commit
(one) and a set of other commits (twos), and computes the merge base of
one and a merge across other commits.

This is mostly useful to figure out the common ancestor when iterating
over heads during an octopus merge.  When making an octopus between
commits A, B, C and D, we first merge tree of A and B, and then try to
merge C with it.  If we were making pairwise merge, we would be recording
the tree resulting from the merge between A and B as a commit, say M, and
then the next round we will be computing the merge base between M and C.

         o---C...*
        /       .
       o---B...M
      /       .
     o---o---A

But during an octopus merge, we actually do not create a commit M.  In
order to figure out that the common ancestor to use for this merge,
instead of computing the merge base between C and M, we can call
merge_bases_many() with one set to C and twos containing A and B.

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

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