midx: write object id fanout chunk

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx.h

@@ -15,6 +15,7 @@ struct multi_pack_index {
 	uint32_t num_objects;
 
 	const unsigned char *chunk_pack_names;
+	const uint32_t *chunk_oid_fanout;
 	const unsigned char *chunk_oid_lookup;
 
 	const char **pack_names;