codereview: use hgapplydiff instead of hgpatch
codereview: use hgapplydiff instead of hgpatch

Depends on https://codereview.appspot.com/7308067

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7312064
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go