Do not fail after calling bisect_auto_next()

As a convenience measure, 'bisect bad' or 'bisect good' automatically
does 'bisect next' when it knows it can, but the result of that test
to see if it can was leaking through as the exit code from the whole
thing, which was bad.  Noticed by Anton Blanchard.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-bisect.sh

@@ -110,7 +110,7 @@ bisect_next_check() {
 }
 
 bisect_auto_next() {
-	bisect_next_check && bisect_next
+	bisect_next_check && bisect_next || :
 }
 
 bisect_next() {