upgrade: avoid a traceback in case of unrecognized revlog
Without this, in case of revlog name not matching any know pattern we would get
a traceback. Raising a clear error seems simpler.
Differential Revision: https://phab.mercurial-scm.org/D11202
from typing import Optional
version: int
def b85encode(text: bytes, pad: Optional[int]) -> bytes: ...
def b85decode(text: bytes) -> bytes: ...