Show-all-refs support in gitlab-workhorse git-http

Per https://gitlab.com/gitlab-org/gitlab-ee/issues/3341#note_43000322

If we're to use git-over-HTTPS in Geo, we need to modify workhorse to take and respect a ShowAllRefs parameter, which will tell git/gitaly to unhide any refs that are normally hidden.

Determining when it's appropriate to flip the switch will be the reponsibility of gitlab-rails. It's conceivable that we might want to use it in non-geo contexts too, so I'll build the supporting code in gitlab-ce.