sequencer: refactor check_todo_list() to work on a todo_list

sequencer: refactor check_todo_list() to work on a todo_list

This refactors check_todo_list() to work on a todo_list to avoid
redundant reads and writes to the disk.  The function is renamed
todo_list_check().  The parsing of the two todo lists is left to the
caller.

As rebase -p still need to check the todo list from the disk, a new
function is introduced, check_todo_list_from_file().  It reads the file
from the disk, parses it, pass the todo_list to todo_list_check(), and
writes it back to the disk.

As get_missing_commit_check_level() and the enum
missing_commit_check_level are no longer needed inside of sequencer.c,
they are moved to rebase-interactive.c, and made static again.

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