Define a built-in attribute macro "binary".

For binary files we would want to disable textual diff
generation and automatic crlf conversion.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

attr.c

@@ -194,6 +194,7 @@ static void free_attr_elem(struct attr_stack *e)
 }
 
 static const char *builtin_attr[] = {
+	"[attr]binary !diff !crlf",
 	NULL,
 };