context: introduce the nullsub() method

Ultimately, this will be used by scmutil.  The subrepo module already imports
it, so it can't import the subrepo module to access the underlying method.

mercurial/context.py

@@ -251,6 +251,9 b' class basectx(object):'
     def sub(self, path):
         return subrepo.subrepo(self, path)
 
+    def nullsub(self, path, pctx):
+        return subrepo.nullsubrepo(self, path, pctx)
+
     def match(self, pats=[], include=None, exclude=None, default='glob',
               listsubrepos=False):
         r = self._repo