In sequencer.c, caller only pass TODO_SQUASH or TODO_FIXUP to
update_squash_messages(), any other command passed in should be
considered as BUG. Replace `return error('unknown command')`
with `BUG('not a FIXUP or SQUASH')`.
Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn> Signed-off-by: Junio C Hamano <gitster@pobox.com>