t4018-diff-funcname: rework negated last expression test

This test used the non-zero exit status of 'git diff' to indicate that a negated funcname pattern, when placed last, was correctly rejected. The problem with this is that 'git diff' always returns non-zero if it finds differences in the files it is comparing, and the files must contain differences in order to trigger the funcname pattern codepath. Instead of checking for non-zero exit status, make sure the expected error message is printed. Signed-off-by: Brandon Casey <drafnel@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>