misc/makerelease: handle git sub-repositories
misc/makerelease: handle git sub-repositories

Also: checkout sub-repos from Mercurial manually
instead of using "go get". (for the 1.4 release)

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/190720043

zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1,zhangg/go-1