add--interactive: remove unused diff colors
When color support was added, we colored the diffs ourselves.
However,
4af756f3 changed this to simply run "git diff-files"
twice, keeping the colored output separately.
This makes the internal diff color variables obsolete with
one exception: when splitting hunks, we have to manually
recreate the fragment for each part of the split. Thus we
keep $fraginfo_color around to do that correctly.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>