vfs: raise NotImplementedError in abstractvfs._auditvfs

This make sure that auditing is implemented.

mercurial/vfs.py

@@ -47,7 +47,7 b' class abstractvfs(object):'
         raise NotImplementedError('attempted instantiating ' + str(type(self)))
 
     def _auditpath(self, path, mode):
-        pass
+        raise NotImplementedError
 
     def tryread(self, path):
         '''gracefully return an empty string for missing files'''