attr: add GIT_ATTR_INDEX "direction"

This instructs attr mechanism, not to look into working .gitattributes
at all. Needed by tools that does not handle working directory, such
as "git archive".

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

attr.h

@@ -33,7 +33,8 @@ int git_checkattr(const char *path, int, struct git_attr_check *);
 
 enum git_attr_direction {
 	GIT_ATTR_CHECKIN,
-	GIT_ATTR_CHECKOUT
+	GIT_ATTR_CHECKOUT,
+	GIT_ATTR_INDEX,
 };
 void git_attr_set_direction(enum git_attr_direction, struct index_state *);