codereview: support mercurial 2.2.1

We explicitly use plainformatter to avoid the
        user's debug setting changing our behavior.
        Fixes #3603.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/6201069

lib/codereview/codereview.py

@@ -3344,6 +3344,10 @@ class FakeMercurialUI(object):
 	def status(self, *args, **opts):
 		pass
 
+	def formatter(self, topic, opts):
+		from mercurial.formatter import plainformatter
+		return plainformatter(self, topic, opts)
+	
 	def readconfig(self, *args, **opts):
 		pass
 	def expandpath(self, *args, **opts):