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.
abort: edit failed: false exited with status 1
transaction abort!
rollback completed
abort: edit failed: false exited with status 1
transaction abort!
rollback completed