diff test: reading a directory as a file need not error out

There is no guarantee that strbuf_read_file must error out for directories. On some operating systems (e.g., Debian GNU/kFreeBSD wheezy), reading a directory gives its raw content: $ head -c5 < / | cat -A ^AM-|^_^@^L$ As a result, 'git diff -O/' succeeds instead of erroring out on these systems, causing t4056.5 "orderfile is a directory" to fail. On some weird OS it might even make sense to pass a directory to the -O option and this is not a common user mistake that needs catching. Remove the test. Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>