container/heap: fix int overflow bug

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7450052

Committer: Russ Cox <rsc@golang.org>
misc/dashboard/builder: make -fail mode faster

Don't check out the repository if we're just going to fail everything.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7453048