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

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang