sequencer: move check_todo_list_from_file() to rebase-interactive.c

sequencer: move check_todo_list_from_file() to rebase-interactive.c

The message contained in `edit_todo_list_advice' (sequencer.c) is
printed after the initial edit of the todo list if it can't be parsed or
if commits were dropped.  This is done either in complete_action() for
`rebase -i', or in check_todo_list_from_file() for `rebase -p'.

Since we want to add this check when editing the list, we also want to
use this message from edit_todo_list() (rebase-interactive.c).  To this
end, check_todo_list_from_file() is moved to rebase-interactive.c, and
`edit_todo_list_advice' is copied there.  In the next commit,
complete_action() will stop using it, and `edit_todo_list_advice' will
be removed from sequencer.c.

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