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: Jonathan Nieder <jrnieder@uchicago.edu> Acked-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>