Use Rugged's TagCollection#create instead of gitlab-shell's Repository#add_tag for better performance

This was originally opened at !1757 by @pcarranza but I changed it to use Rugged instead of gitlab_git, following @DouweM's request.

Once this is merged, https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/26 will be mergeable too.

See merge request !3745