context: add overlayfilectx.cmp()

Differential Revision: https://phab.mercurial-scm.org/D1058

mercurial/context.py

@@ -2098,6 +2098,9 b' class overlayworkingfilectx(workingfilec'
         self._parent = parent
         self._path = path
 
+    def cmp(self, fctx):
+        return self.data() != fctx.data()
+
     def ctx(self):
         return self._parent