Default kamal rollback to prior deploy

[motdotla]

The typical rollback flow is:

1. Oh shoot, my deploy succeeded but my container is down
2. Run kamal app containers
3. Identify 2nd from top container commit-hash
4. Run kamal rollback commit-hash
5. Yay, my app is back up

It would be nice if kamal rollback (without any arguments) did the above automatically. This is what heroku rollback does.

[dhh]

I like that. Please do investigate.