Geo might not sync custom refs

Custom refs are used by GitLab in for example MergeRequestController#show, but also end users might use git-notes. Custom refs need to be explicitly marked by a client to signal they want to sync those too.

Gitlab::Shell#fetch_remote and/or Gitaly seems not to use a ref spec that includes everything we might be interested in.

I might, and probably am, wrong, but opening an issue to increase visibility. :)