git-svn: get color config from --get-colorbool

git-svn: get color config from --get-colorbool

git-config recently learned a --get-colorbool option. By
using it, we will get the same color=auto behavior that
other git commands have.

Specifically, this fixes the case where "color.diff = true"
meant "always" in git-svn, but "auto" in other programs.

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