cmd/go: "go get" don't ignore git default branch

cmd/go: "go get" don't ignore git default branch

Any Git branch can be the default branch not only master. Removing
hardwired 'checkout master', and using 'checkout {tag}' is the best
choice. It works with and without a master branch. Furthermore it
resolves the Github default branch issue. Changing Github default
branch is effectively changing HEAD.

Fixes #9032

Change-Id: I19a1221bcefe0806e7556c124c6da7ac0c2160b5
Reviewed-on: https://go-review.googlesource.com/5312
Reviewed-by: Russ Cox <rsc@golang.org>