sequencer: change complete_action() to use the refactored functions

sequencer: change complete_action() to use the refactored functions

complete_action() used functions that read the todo-list file, made some
changes to it, and wrote it back to the disk.

The previous commits were dedicated to separate the part that deals with
the file from the actual logic of these functions.  Now that this is
done, we can call directly the "logic" functions to avoid useless file
access.

The parsing of the list has to be done by the caller.  If the buffer of
the todo list provided by the caller is empty, a `noop' command is
directly added to the todo list, without touching the buffer.

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