Okay, that was a little too aggressive.

Okay, that was a little too aggressive.

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.