gitserver: Use WARN log-level when cleaning up LRU repos
Created by: mrnugget
Slack thread with context: https://sourcegraph.slack.com/archives/CHPC7UX16/p1566316586140100?thread_ts=1565708902.099900&cid=CHPC7UX16
We've had at least one customer run into repo-updater
seemingly cloning repositories in an endless loop. It turned out that
gitserver
was at the same time cleaning up LRU repos because there was
not enough disk space.
The log message was not visible to the user in the default configuration. With this change we make it visible.