topics: fix auto-publish=abort with servers publishing bare branches

Before this change, servers that automatically publish bare branches were
detected as "non publishing" and the check for automatic publishing on push
assumed nothing was to be published. Now we signal the server-side config to
clients via a new capability so that they can adjust their behavior.
7 jobs for topic/stable/fix-publish in 5 minutes and 17 seconds (queued for 3 seconds)