This will mean that sub-repositories won't get built against the
release branch. They are often not compatible because the subrepos
often run ahead of the current release (e.g. go.tools is using
new additions to go/ast, and go.net is using new things in syscall)
so there's little point in checking them against cherrypick commits
when they'll be tested against those commits on tip anyway.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/12001043