basectx: change _node to node() in hex

This allows a child class to overload the node() function and still share the
same code for hex().

mercurial/context.py

@@ -43,7 +43,7 b' class basectx(object):'
     def node(self):
         return self._node
     def hex(self):
-        return hex(self._node)
+        return hex(self.node())
 
 class changectx(basectx):
     """A changecontext object makes access to data related to a particular