Prior to this change, a feature flag could be used to disable this
feature. Now all requests go through Gitaly's OperationService.

Closes https://gitlab.com/gitlab-org/gitaly/issues/540

When vendoring `Gitlab::Git` again in Gitaly, this implemenation will be
gone, but this is readded there through:
https://gitlab.com/gitlab-org/gitaly/merge_requests/717