repo-updater: populate git fetch scheduler on startup
Created by: keegancsmith
The git fetch scheduler discovers what repos to fetch/clone by reading the Synced and SubsetSynced channels from the Syncer. On repo-updater startup it will only start running once the first Sync is complete (this is when it first discovers repos). On large installations a sync can take a while. This change allows the scheduler to start syncing the pre-existing repos (common case) on startup.
Part of https://github.com/sourcegraph/sourcegraph/issues/11684