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.
*** failed to import extension badext: invalid syntax (badext.py, line 1)hg help [COMMAND]show help for a command, extension, or list of commands