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>