t4018-diff-funcname: rework negated last expression test

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>