goinstall/download.go: Add checkout concept and helper functions to clean and fix implementation (Issue 1265)
goinstall/download.go: Add checkout concept and helper functions to clean and fix implementation (Issue 1265)

R=rsc, niemeyer, adg
CC=golang-dev
http://codereview.appspot.com/3536041

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go