cmd/go: add built in git mode for GOAUTH
This CL adds support for git as a valid GOAUTH command.
Improves on implementation in cmd/auth/gitauth/gitauth.go
This follows the proposed design in
https://golang.org/issues/26232#issuecomment-
461525141
For #26232
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Change-Id: I07810d9dc895d59e5db4bfa50cd42cb909208f93
Reviewed-on: https://go-review.googlesource.com/c/go/+/605275
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>