merge.mergestate: set merge driver state to 's' if there's none present

This allows mergestate.mdstate() to return 's' if there's nothing to be done on
the merge driver end.

mercurial/merge.py

@@ -89,8 +89,11 b' class mergestate(object):'
         if node:
             self._local = node
             self._other = other
-        self._mdstate = 'u'
         self._readmergedriver = None
+        if self.mergedriver:
+            self._mdstate = 's'
+        else:
+            self._mdstate = 'u'
         shutil.rmtree(self._repo.join('merge'), True)
         self._dirty = False
 
@@ -103,10 +106,10 b' class mergestate(object):'
         self._state = {}
         self._local = None
         self._other = None
-        self._mdstate = 'u'
         if 'otherctx' in vars(self):
             del self.otherctx
         self._readmergedriver = None
+        self._mdstate = 's'
         records = self._readrecords()
         for rtype, record in records:
             if rtype == 'L':