https://gitlab.com/gitlab-org/gitlab-git-http-server
This change introduces the GITLAB_GRACK_AUTH_ONLY environment
variable. When set, Grack requests to GitLab will only respond with
the user's GL_ID (if the request is OK) or an error. This allows
gitlab-git-http-server to use the main GitLab application as an
authentication and authorization backend.
If we like how this works we should drop the GITLAB_GRACK_AUTH_ONLY
variable at some point in the future.
