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.

#require emacs
  $ emacs -q -no-site-file -batch -l $TESTDIR/../contrib/hg-test-mode.el \
  >  -f ert-run-tests-batch-and-exit
  Running 1 tests (*) (glob)
     passed  1/1  hg-test-mode--compilation-mode-support* (glob)
  
  Ran 1 tests, 1 results as expected* (glob)