sequencer: refactor how original todo list lines are accessed

sequencer: refactor how original todo list lines are accessed

Previously, we did a lot of arithmetic gymnastics to get at the line in
the todo list (as stored in todo_list.buf). This might have been fast,
but only in terms of execution speed, not in terms of developer time.

Let's refactor this to make it a lot easier to read, and hence to
reason about the correctness of the code. It is not performance-critical
code anyway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>