Suggest using url.$URL.insteadOf to speed up "go get"

By setting the url.https://gwt.googlesource.com/.insteadOf option to
git://gwt.googlesource.com/, when "go get" tries to first download
buildglue from git://gwt.googlesource.com/buildglue, git will
automatically rewrite the URL to use https:// and avoid the
significant delay.

The url.$URL.insteadOf option is documented in the git-fetch manual.

Change-Id: I50ab5b669d5026f011c35c1a19ec0c6dc1b37762