Fix huge bug w/ assim'd merge commits w/ diff Main trees #hugeBug #typo

Now that I think of it, the positive test case that assimilated merge
commits whose parents have the same Main trees uses that Main tree isn't
important, what's important is negative test cases that
git-subhistory doesn't screw you over, like in this case where it just
silently deletes yet-another-Main-thing (of course it's still in
history, but still).