Bisect reset: remove bisect refs that may have been packed.

If refs were ever packed in the middle of bisection, the bisect
refs were not removed from the "packed-refs" file.

This patch fixes this problem by using "git update-ref -d $ref $hash"
in "bisect_clean_state".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>