pack-bitmap: switch hard-coded constants to the_hash_algo

Switch two hard-coded uses of 20 to references to the_hash_algo.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-bitmap.c

@@ -138,7 +138,7 @@ static int load_bitmap_header(struct bitmap_index *index)
 {
 	struct bitmap_disk_header *header = (void *)index->map;
 
-	if (index->map_size < sizeof(*header) + 20)
+	if (index->map_size < sizeof(*header) + the_hash_algo->rawsz)
 		return error("Corrupted bitmap index (missing header data)");
 
 	if (memcmp(header->magic, BITMAP_IDX_SIGNATURE, sizeof(BITMAP_IDX_SIGNATURE)) != 0)
@@ -157,7 +157,7 @@ static int load_bitmap_header(struct bitmap_index *index)
 				"(Git requires BITMAP_OPT_FULL_DAG)");
 
 		if (flags & BITMAP_OPT_HASH_CACHE) {
-			unsigned char *end = index->map + index->map_size - 20;
+			unsigned char *end = index->map + index->map_size - the_hash_algo->rawsz;
 			index->hashes = ((uint32_t *)end) - index->pack->num_objects;
 		}
 	}