Test that we notice the empty files from the second parent during a merge
This didn't work with 0.9.1, but was fixed by
05257fd28591.
revlog.py: always return tuples from parents and parentrevs
There's at least one place doing parentrevs(rev) != (rev-1, -1),
which is always true with revlog v0, since parentrevs returns a list
in this case.