Stale worktrees pointing to invalid HEAD should be cleaned up right away

The Cleanup RPC attempts to remove stale worktrees after 6 hours, but a customer ran into an issue where there were worktrees pointing to a new branch (0000000000000000000000000000000000000000) were preventing pushes from going through. We ran git worktree prune, and it fixed the problem.

Perhaps we might want to run git worktree prune along with the Cleanup.