UX improvement for HEAD pointing to non-existent snapshots or inconsistent worktree

[peterwaller-arm]

#90 highlighted that things can get a bit confusing if users move files around themselves. This behaviour doesn't seem unreasonable to me in some circumstances. For example, moving or copying an elfshaker_data with a HEAD file to an empty directory seems supportable if we special case the 'worktree is empty' case, we could just ignore the HEAD. I can't think of a case where this would be unwanted behaviour.