dagop: don't import nullrev from .node twice

See 59fa3890d40a.

Differential Revision: https://phab.mercurial-scm.org/D12120

mercurial/dagop.py

@@ -9,7 +9,6 b' from __future__ import absolute_import'
 
 import heapq
 
-from .node import nullrev
 from .thirdparty import attr
 from .node import nullrev
 from . import (