upgrade: correct implementation of improvement.__ne__

The "not" operator binds more closely than "==":
>>> not False == False
False

mercurial/upgrade.py

@@ -142,7 +142,7 b' class improvement(object):'
         return self.name == other.name
 
     def __ne__(self, other):
-        return not self == other
+        return not (self == other)
 
     def __hash__(self):
         return hash(self.name)