Cleanup RPC prunes disconnected work trees

Changes the Cleanup RPC behavior to run git prune -expire=6h. Previously, the RPC would manually delete stale (i.e. older than 6 hours) work tree administrative directories. By running the new command, we will also be able to remove administrative work tree directories that are in a "disconnected" state (i.e. the administrative files refer to a work tree directory that no longer exists). These disconnected work trees will be cleaned up regardless of how old they are.

Closes #1551 (closed)