test: add test for a former race resulting in bad dirstate
In 6.0 this used to result in the size being stored in the dirstate is wrong.
This was fixed by other change to the mtime gathering logic.
Differential Revision: https://phab.mercurial-scm.org/D11749
from typing import Optional
version: int
def b85encode(text: bytes, pad: Optional[int]) -> bytes: ...
def b85decode(text: bytes) -> bytes: ...