mergestate: simplify reset(), knowing that `other` and `node` go together

There's only one caller of `reset()` that passes any arguments at all,
and that originates from `merge.py:1371`. That code always passes
values for both `node` and `other`.

Differential Revision: https://phab.mercurial-scm.org/D9032

mercurial/mergestate.py

@@ -202,15 +202,12 b' class mergestate(object):'
     def reset(self, node=None, other=None, labels=None):
         self._state = {}
         self._stateextras = collections.defaultdict(dict)
-        self._local = None
-        self._other = None
+        self._local = node
+        self._other = other
         self._labels = labels
         for var in ('localctx', 'otherctx'):
             if var in vars(self):
                 delattr(self, var)
-        if node:
-            self._local = node
-            self._other = other
         self._readmergedriver = None
         if self.mergedriver:
             self._mdstate = MERGE_DRIVER_STATE_SUCCESS