This will delete branches merged into the current branch.
This uses `-d` and is therefore safer than the current script (which uses `-D`).
IMO, this mitigates the need to prompt.
It won't delete master. If you have a ligitimate reasn to delete master,
you probably should use delete-branch directly.
It works for multiple branches and it asks for confirmation each time.
-f|--force supresses confirmation
You can answer 'a' (all) when prompted, which means you won't be asked
again.
