context: narrow down filter special case in filectx.cmp()

cmp via filelog when encode filters are present, but only when
actually comparing to the working directory.

mercurial/context.py

@@ -357,11 +357,12 b' class filectx(object):'
 
         returns True if different than fctx.
         """
-        if not self._repo._encodefilterpats and self.size() != fctx.size():
+        if (fctx._filerev is None and self._repo._encodefilterpats
+            or self.size() == fctx.size()):
+            return self._filelog.cmp(self._filenode, fctx.data())
+
             return True
 
-        return self._filelog.cmp(self._filenode, fctx.data())
-
     def renamed(self):
         """check if file was actually renamed in this changeset revision