util: add __ne__ to filestat class for consistency

This is follow up for ca4065028e00, which introduced filestat class.

mercurial/util.py

@@ -1455,6 +1455,9 b' class filestat(object):'
         except AttributeError:
             return False
 
+    def __ne__(self, other):
+        return not self == other
+
 class atomictempfile(object):
     '''writable file object that atomically updates a file