bisect: fix missing "exit"

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>

Author: chriscool

git-bisect.sh

@@ -455,7 +455,7 @@ bisect_next() {
 	good=$(git for-each-ref --format='^%(objectname)' \
 		"refs/bisect/good-*" | tr '\012' ' ') &&
 	skip=$(git for-each-ref --format='%(objectname)' \
-		"refs/bisect/skip-*" | tr '\012' ' ') &&
+		"refs/bisect/skip-*" | tr '\012' ' ') || exit
 
 	# Maybe some merge bases must be tested first
 	check_good_are_ancestors_of_bad "$bad" "$good" "$skip"