rebase-interactive: use todo_list_write_to_file() in edit_todo_list()

rebase-interactive: use todo_list_write_to_file() in edit_todo_list()

Just like complete_action(), edit_todo_list() used a
function (transform_todo_file()) that read the todo list from the disk
and wrote it back, resulting in useless disk accesses.

This changes edit_todo_list() to call directly todo_list_write_to_file()
instead.

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>