Check to see given bad/good/skip sets are valid commit and to exit
otherwise was broken by 6a54d97 (bisect: remove "checkout_done" variable
used when checking merge bases, 2008-09-06).
Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>