cmd/go/testdata/vcstest/git: use git commands that work on older git versions
On older versions of git (such as 2.25.1), `git branch -M master`
fails after `git init` - use `git checkout -b master` instead, which
works across multiple versions.
Fixes tests on the rather outdated linux/mips* builders.
Change-Id: Id86ec82b37a8d2c3131f449276b13989d097d106
Reviewed-on: https://go-review.googlesource.com/c/go/+/729600
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>