localrepo: migrate to context manager for changing dirstate parents

mercurial/localrepo.py

@@ -850,7 +850,7 b' class localrepository(object):'
         return self[changeid]
 
     def setparents(self, p1, p2=nullid):
-        self.dirstate.beginparentchange()
+        with self.dirstate.parentchange():
         copies = self.dirstate.setparents(p1, p2)
         pctx = self[p1]
         if copies:
@@ -864,7 +864,6 b' class localrepository(object):'
             for f, s in sorted(self.dirstate.copies().items()):
                 if f not in pctx and s not in pctx:
                     self.dirstate.copy(None, f)
-        self.dirstate.endparentchange()
 
     def filectx(self, path, changeid=None, fileid=None):
         """changeid can be a changeset revision, node, or tag.