dirstate: make a conditionnal easier to read in `setparents`

By grabing `nullid` in the local space, we get the conditionnal to fit in one
line.

Differential Revision: https://phab.mercurial-scm.org/D11427

mercurial/dirstate.py

@@ -387,10 +387,8 b' class dirstate(object):'
             self._origpl = self._pl
         self._map.setparents(p1, p2)
         copies = {}
-        if (
-            oldp2 != self._nodeconstants.nullid
-            and p2 == self._nodeconstants.nullid
-        ):
+        nullid = self._nodeconstants.nullid
+        if oldp2 != nullid and p2 == nullid:
             candidatefiles = self._map.non_normal_or_other_parent_paths()
 
             for f in candidatefiles: