sequencer: refactor skip_unnecessary_picks() to work on a todo_list

sequencer: refactor skip_unnecessary_picks() to work on a todo_list

This refactors skip_unnecessary_picks() to work on a todo_list.  As this
function is only called by complete_action() (and thus is not used by
rebase -p), the file-handling logic is completely dropped here.

Instead of truncating the todo list’s buffer, the items are moved to
the beginning of the list, eliminating the need to reparse the list.
This also means its buffer cannot be directly written to the disk.

rewrite_file() is then removed, as it is now unused.

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