If the todo list generated by `--make-script` is empty,
complete_action() writes a noop, but if it has only commented-out
commands, it will abort with the message "Nothing to do", and does not
launch the editor. This adds a new test to ensure that
complete_action() behaves this way.
Signed-off-by: Alban Gruin <alban.gruin@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>