t6030 (bisect): work around Mac OS X "ls"

t6030-bisect-porcelain.sh relies on "ls" exiting with nonzero
status when asked to list nonexistent files.  Unfortunately,
/bin/ls on Mac OS X 10.3 exits with exit code 0.  So look at
its output instead.
Signed-off-by: NJonathan Nieder <jrnieder@uchicago.edu>
Acked-by: NChristian Couder <chriscool@tuxfamily.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t6030-bisect-porcelain.sh

@@ -76,7 +76,7 @@ test_expect_success 'bisect fails if given any junk instead of revs' '
 	test_must_fail git bisect start foo $HASH1 -- &&
 	test_must_fail git bisect start $HASH4 $HASH1 bar -- &&
 	test -z "$(git for-each-ref "refs/bisect/*")" &&
-	test_must_fail ls .git/BISECT_* &&
+	test -z "$(ls .git/BISECT_* 2>/dev/null)" &&
 	git bisect start &&
 	test_must_fail git bisect good foo $HASH1 &&
 	test_must_fail git bisect good $HASH1 bar &&