templatefilters: add missing import of _ stable

templatefilters: add missing import of _

--- a/mercurial/templatefilters.py	Thu Feb 28 13:45:18 2013 +0100
+++ b/mercurial/templatefilters.py	Thu Feb 28 13:55:00 2013 +0100
@@ -5,6 +5,7 @@
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
+from i18n import _
 import cgi, re, os, time, urllib
 import encoding, node, util, error
 import hbisect