vfs: add a `_auditpath` to abstract vfs

We are about to make `rename` audit path. Since rename lives in the
`abstractvfs` layer, we need it to be aware of auditing to some extent.

The default implementation is no-op because multiple existing vfs are not using
auditing at all right now (eg: fncachevfs).

mercurial/vfs.py

@@ -46,6 +46,9 b' class abstractvfs(object):'
         '''Prevent instantiation; don't call this from subclasses.'''
         raise NotImplementedError('attempted instantiating ' + str(type(self)))
 
+    def _auditpath(self, path, mode):
+        pass
+
     def tryread(self, path):
         '''gracefully return an empty string for missing files'''
         try: