diff-highlight: use test_decode_color in tests

diff-highlight: use test_decode_color in tests

The diff-highlight tests use raw color bytes when comparing expected and
actual output. Let's use test_decode_color, which is our usual technique
in other tests. It makes reading test output diffs a bit easier, since
you're not relying on your terminal to interpret the result (or worse,
interpreting characters yourself via "cat -A").

This will also make it easier to add tests with new colors/attributes,
without having to pre-define the byte sequences ourselves.

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