builtin rebase: support --edit-todo and --show-current-patch

builtin rebase: support --edit-todo and --show-current-patch

While these sub-commands are very different in spirit, their
implementation is almost identical, so we convert them in one go.

And since those are the last sub-commands that needed to be converted,
now we can also turn that `default:` case into a bug (because we should
now handle all the actions).

Signed-off-by: Pratik Karki <redacted>
Signed-off-by: Junio C Hamano <redacted>