Worktree clean up

ZD: https://gitlab.zendesk.com/agent/tickets/124042

In some situations, it has been observed that there are two "worktree" related directories in a repository's folder:

gitlab-worktree
worktrees

It was observed that both directories were populated with rebase-[0-9]+ named rebase directories. Manually running house keeping does not remove the stale rebase folders in gitlab-worktree.

Some concerns:

Why do we have multiple naming conventions for where we put rebase folders?
Why does housekeeping not seem to work on gitlab-worktree?

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