t: use test_decode_color rather than literal ANSI codes

t: use test_decode_color rather than literal ANSI codes

When we put literal ANSI terminal codes into our test
scripts, it makes diffs on those scripts hard to read (the
colors may be indistinguishable from diff coloring, or in
the case of a reset, may not be visible at all).

Some scripts get around this by including human-readable
names and converting to literal codes with a git-config
hack. This makes the actual code diffs look OK, but test_cmp
output suffers from the same problem.

Let's use test_decode_color instead, which turns the codes
into obvious text tags.

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