sequencer: refactor transform_todos() to work on a todo_list

sequencer: refactor transform_todos() to work on a todo_list

This refactors transform_todos() to work on a todo_list.  The function
is renamed todo_list_transform().

As rebase -p still need to check the todo list from the disk, a new
function is introduced, transform_todo_file().  It is still used by
complete_action() and edit_todo_list() for now, but they will be
replaced in a future commit.

todo_list_transform() is not a static function, because it will be used
by edit_todo_list() from rebase-interactive.c in a future commit.

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