Now we only prune out a commit if it has exactly one remaining parent and
that parent's tree is identical to ours.
But I also changed the test to create the initial "-s ours" merge in one
step instead of two, and that merge can be eliminated since one of its
parents doesn't affect the subdir at all, and is thus deleted.
Now we cut out a commit if any of its parents had the same tree; just use
that parent in its place. This makes the history look nice, but I don't
think it's quite right...
The idea is to join the new split branch back into this one, so future
splits can append themselves to the old split branch. We mark the split
branch's history in our merge commit, so we can pull it back out later.
