diff options: add --color

This patch is a slightly adjusted version of Junio's patch:
http://www.gelato.unsw.edu.au/archives/git/0604/19354.html

However, instead of using a config variable, this patch makes it available
as a diff option.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -32,7 +32,8 @@ struct diff_options {
 		 full_index:1,
 		 silent_on_remove:1,
 		 find_copies_harder:1,
-		 summary:1;
+		 summary:1,
+		 color_diff:1;
 	int context;
 	int break_opt;
 	int detect_rename;