mergestate: consistently set variables to None

Some code branches and exceptional circumstances such as empty
mergestate files could result in mergestate._local and
mergestate._other not being defined or reset to None. These variables
are now correctly set to None when they should be.

mercurial/merge.py

@@ -55,6 +55,8 b' class mergestate(object):'
 
     def reset(self, node=None, other=None):
         self._state = {}
+        self._local = None
+        self._other = None
         if node:
             self._local = node
             self._other = other
@@ -68,6 +70,8 b' class mergestate(object):'
         of on disk file.
         """
         self._state = {}
+        self._local = None
+        self._other = None
         records = self._readrecords()
         for rtype, record in records:
             if rtype == 'L':