sequencer: make three functions and an enum from sequencer.c public

sequencer: make three functions and an enum from sequencer.c public

This makes rebase_path_todo(), get_missing_commit_check_level(),
write_message() and the enum check_level accessible outside sequencer.c,
renames check_level to missing_commit_check_level, and prefixes its
value names by MISSING_COMMIT_ to avoid namespace pollution.

This function and this enum will eventually be moved to
rebase-interactive.c and become static again, so no special attention
was given to the naming.

This will be needed for the rewrite of append_todo_help() from shell to
C, as it will be in a new library source file, rebase-interactive.c.

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