diff: Support both attributes and colors

diff: Support both attributes and colors

Make it possible to set both colors and a attribute for diff colors.
Background colors are supported too.

Syntax is now:

	[attr] [fg [bg]]
	[fg [bg]] [attr]

Empty value is same as "normal normal", ie use default colors.  The new
syntax is backwards compatible.

Signed-off-by: Timo Hirvonen <tihirvon@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>