Suppose you have this topology, and you are trying to make an octopus
across A, B and C (you are at C and merging A and B into your branch).
The protoccol between "git merge" and merge strategies is for the former
to pass common ancestor(s), '--' and then commits being merged.

git-merge-octopus does not produce the final merge in one-go.  It
iteratively produces pairwise merges.  So the first step might be to come
up with a merge between B and C:

               o---o---o---o---C
              /                 :
             /   o---o---o---B..(M)
            /   /
        ---1---2---o---o---o---A

and for that, "1" is used as the merge base, not because it is the base
across A, B and C but because it is the base between B and C.  For this
merge, A does not matter.

I drew M in parentheses and lines between B and C to it in dotted line
because we actually do _not_ create a real commit --- the only thing we
need is a tree object,...