Came here to praise jj as well. More than just "git-compatible", its backing store is literally git, which means that you can transparently collab with existing git repos to your heart's desire.

Other interesting ideas it implements:

    - Working copy always comitted,  
    - All repo operations are version controlled, which enables painless undo etc.  
    - First-class conflicts, just like pijul, so no merge/rebase/etc ops every fail  
    - Auto-rebase of downstream dependent branches when doing rebase etc.

Also, the dev is really active and helpful. Apparently, Google let him make the project his full time job.