Create header for pretty.c to make formatting interface more structured.
This is a middle point, this file would be merged further with other
files which contain formatting stuff.
Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>