t4015 contains many color-related tests which need to
override the "is stdout a tty" check. They do so by setting
the color.diff config, but we can accomplish the same with
the --color option. Besides being shorter to type, switching
will prepare us for upcoming changes to "always" when see it
in config.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>