t/.gitattributes: ignore whitespace in chainlint expect files

t/.gitattributes: ignore whitespace in chainlint expect files

The ".expect" files in t/chainlint/ are snippets of expected output from
the chainlint script, and do not necessarily conform to our usual code
style. Especially with the recent change to retain line numbers, blank
lines in the input script end up with trailing whitespace as we print
"3 " for line 3, for example. The point of these files is to match the
output verbatim, so let's not complain about the trailing spaces.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>