R1:f3e154eb02c7

Allow "inactive" repositories to be read over SSH for cluster sync

Summary:
Fixes T13192. See PHI1015. When you deactivate a repository, we currently stop serving it.

This creates a problem for intracluster sync, since new nodes can't sync it. If nothing else, this means that if you "ship of theseus" your cluster and turn nodes over one at a time, you will eventually lose the entire repository. Since that's clearly a bad outcome, support sync.

Test Plan:
Testing this requires a "real" cluster, so I…