builtin/pack-objects: make hash agnostic

Avoid hard-coding a hash size, instead preferring to use the_hash_algo.

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

builtin/pack-objects.c

@@ -879,7 +879,7 @@ static void write_reused_pack_one(size_t pos, struct hashfile *out,
 			len = encode_in_pack_object_header(header, sizeof(header),
 							   OBJ_REF_DELTA, size);
 			hashwrite(out, header, len);
-			hashwrite(out, base_sha1, 20);
+			hashwrite(out, base_sha1, the_hash_algo->rawsz);
 			copy_pack_data(out, reuse_packfile, w_curs, cur, next - cur);
 			return;
 		}