gitattributes: disable blank-at-eof errors for clar test expectations

The clar unit testing framework carries a couple of files that contain expected output for its self-tests. Some of these files expectedly end with a blank line at the end of the file, which Git would consider to be a whitespace error by default. Teach our gitattributes to ignore those errors. Suggested-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>