The proxy filter_prefix cli option is broken

[LMG]

This option adds a prefix to every filter which is requested through the git protocol. It does not add this filter in the graphql API, nor does it add it to the background filtering, rendering this background filtering mostly useless. We would need to apply discover_filter_candidates to the result of applying this prefix filter (as workspaces are only visible after this prefix filter has been applied anyhow).