diff.c: second war on whitespace.

This adds DIFF_WHITESPACE color class (default = reverse red) to
colored diff output to let you catch common whitespace errors.

 - trailing whitespaces at the end of line
 - a space followed by a tab in the indent

Signed-off-by: Junio C Hamano <junkio@cox.net>

diff.h

@@ -86,6 +86,7 @@ enum color_diff {
 	DIFF_FILE_OLD = 4,
 	DIFF_FILE_NEW = 5,
 	DIFF_COMMIT = 6,
+	DIFF_WHITESPACE = 7,
 };
 const char *diff_get_color(int diff_use_color, enum color_diff ix);