fsck: factor out msg_id_info[] lazy initialization code

This array will be used by some other function than parse_msg_id() in
the following commit. Factor out this prep code so it could be called
from that one.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>