largefiles: handle logging from outside the repo
It's probably possible to refactor so that the 'if m._cwd' check isn't
necessary, but the False case is the typical case (i.e. run from the root of the
repo), and simpler to read.
An exact path to a largefile from outside the repo was previously ignored.
match.rel('.hglf') will handle figuring out both the correct '../' length to the
standin directory if inside the repo, or path/to/repo from outside, at the cost
of a pconvert() to keep the patterns using '/' on Windows.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.