format-patch: ignore ui.color

format-patch: ignore ui.color

commit c9bfb953 (want_color: automatically fallback to color.ui,
2011-08-17) introduced a regression where format-patch produces colorized
patches when color.ui is set to "always".

In f3aafa4 (Disable color detection during format-patch, 2006-07-09),
git_format_config was taught to intercept diff.color to avoid passing it
down to git_log_config and later, git_diff_ui_config.

Teach git_format_config to intercept color.ui in the same way.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>