log: fix handling of root (or empty) path provided by matcher (issue6478)
Since 27d6956d386b "match: use '' instead of '.' for root directory",
'.' should be translated to ''. We can't blame repo.file() about this because
an empty string is invalid as a file path, but I found at least two callers
(_makematcher() and revset.filelog()) would crash because of this path.
So let's make repo.file() accept an empty string. path == b'/' wouldn't
work on Python 3 anyways.