lib/codereview: suggest argument to hg mail / hg submit
lib/codereview: suggest argument to hg mail / hg submit

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/6924050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go