t3005: do not assume a particular order of stdout and stderr of git-ls-files

There is no guarantee that stderr is flushed before stdout when both
channels are redirected to a file. Check the channels using independent
files.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>