Python 3.8 can raise ValueError on attempt of an I/O operation
against an illegal path. This was causing test-remotefilelog-gc.t
to fail on Python 3.8.
This commit teaches repository opening to handle ValueError
and re-raise an Abort on failure.
An arguably better solution would be to implement this logic
in the vfs layer. But that seems like a bag of worms and I don't
want to go down that rabbit hole. Until users report uncaught
ValueError exceptions in the wild, I think it is fine to patch
this at the only occurrence our test harness is finding it.