Gitaly cleanup
Some cleanup in Gitaly code.
- assume that gitlab-rails sends GitalyAddress. Valid because of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10447
- assume that gitlab-rails sends a fully populated Repository message. Valid because of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10343
- use pre-built Repository message from API responses. This is something we missed in !143 (merged)