proxy-vfs: also proxy the `audit` attribute
In the previous changeset, we had to do a little dance to access the useful
`audit` attribute. We now provide a proper accessors to it.
We don't update the code in `perf.py` because it has to remain compatible with
older version of Mercurial. This will just be nicer in the future.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.