Enable gettext translations

The new demandimport eliminates most of the gettext performance hit of
importing gettext, which was doubling the run time for 'hg'.

Now, with 'hg tip':

dummy: .09530s no translations: .09638s translations: .09755s

mercurial/i18n.py

@@ -7,9 +7,7 b' This software may be used and distribute'
 of the GNU General Public License, incorporated herein by reference.
 """
 
-# the import from gettext is _really_ slow
-# for now we use a dummy function
-gettext = lambda x: x
-#import gettext
-#t = gettext.translation('hg', '/usr/share/locale', fallback=1)
-#gettext = t.gettext
+import gettext
+t = gettext.translation('hg', fallback=1)
+gettext = t.gettext
+_ = gettext