mergestate: implement unresolvedcount() in terms of unresolved()

This simplifies the method slightly. It does create a full list of
paths while doing so, but it's not a lot of data anyway (besides, I
would think references to strings are no larger than (references to?)
True).

mercurial/merge.py

@@ -567,8 +567,7 b' class mergestate(object):'
 
     def unresolvedcount(self):
         """get unresolved count for this merge (persistent)"""
-        return len([True for f, entry in self._state.iteritems()
-                    if entry[0] == 'u'])
+        return len(list(self.unresolved()))
 
     def actions(self):
         """return lists of actions to perform on the dirstate"""