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.

mercurial/vfs.py

@@ -616,6 +616,10 b' class proxyvfs(abstractvfs):'
     def options(self, value):
         self.vfs.options = value
 
+    @property
+    def audit(self):
+        return self.vfs.audit
+
 
 class filtervfs(proxyvfs, abstractvfs):
     '''Wrapper vfs for filtering filenames with a function.'''