context: use wdirhex constant instead of calculating it

Differential Revision: https://phab.mercurial-scm.org/D6143

mercurial/context.py

@@ -21,7 +21,7 b' from .node import ('
     nullrev,
     short,
     wdirfilenodeids,
-    wdirid,
+    wdirhex,
 )
 from . import (
     dagop,
@@ -1324,7 +1324,7 b' class workingctx(committablectx):'
         return self._repo.dirstate[key] not in "?r"
 
     def hex(self):
-        return hex(wdirid)
+        return wdirhex
 
     @propertycache
     def _parents(self):