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.
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https://www.mercurial-scm.org/wiki/ContributingChanges
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273ce12ad8f1` will report some style violations on a very old commit).