treedirstate: dirstatemap.identity should be a property

The dirstatemap identity attribute should be a property, as the dirstate
accesses it this way.  Currently it is using the bound method as if it were the
identity, which it is not.

Differential Revision: https://phab.mercurial-scm.org/D1680

hgext3rd/treedirstate.py

@@ -384,6 +384,7 @@ class treedirstatemap(object):
                 return None
         return _overlaydict(lookup)
 
+    @property
     def identity(self):
         if self._identity is None:
             self.read()