rev-list: pass diffopt->use_colors through to pretty-print

rev-list: pass diffopt->use_colors through to pretty-print

When rev-list pretty-prints a commit, it creates a new
pretty_print_context and copies items from the rev_info
struct. We don't currently copy the "use_color" field,
though. Nobody seems to have noticed because the only part
of pretty.c that cares is the %C(auto,...) placeholder, and
presumably not many people use that with the rev-list
plumbing (as opposed to with git-log).

It will become more noticeable in a future patch, though,
when we start treating all user-format colors as auto-colors
(in which case it would become impossible to format colors
with rev-list, even with --color=always).

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