util: add __ne__ to filestat class for consistency

util: add __ne__ to filestat class for consistency

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

--- a/mercurial/util.py	Sat Apr 16 16:01:24 2016 -0700
+++ b/mercurial/util.py	Fri Jun 03 00:44:20 2016 +0900
@@ -1455,6 +1455,9 @@
         except AttributeError:
             return False
 
+    def __ne__(self, other):
+        return not self == other
+
 class atomictempfile(object):
     '''writable file object that atomically updates a file