diff-highlight: test color config

diff-highlight: test color config

We added configurable colors long ago in bca45fbc1f (diff-highlight:
allow configurable colors, 2014-11-20), but never actually tested it.
Since we'll be touching the color code in a moment, this is a good time
to beef up the tests.

Note that we cover both the highlight/reset style used by the default
colors, as well as the normal/highlight style added by that commit
(which was previously totally untested).

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