diff -r 35985d407d49 -r e9bf3e132ea9 mercurial/formatter.py
--- a/mercurial/formatter.py	Sat May 27 17:40:18 2017 +0900
+++ b/mercurial/formatter.py	Sat May 27 17:44:26 2017 +0900
@@ -207,6 +207,10 @@
         if self._item is not None:
             self._showitem()
 
+def nullformatter(ui, topic):
+    '''formatter that prints nothing'''
+    return baseformatter(ui, topic, opts={}, converter=_nullconverter)
+
 class _nestedformatter(baseformatter):
     '''build sub items and store them in the parent formatter'''
     def __init__(self, ui, converter, data):