bundle: detect hash algorithm when reading refs

Much like with the dumb HTTP transport, there isn't a way to explicitly
specify the hash algorithm when dealing with a bundle, so detect the
algorithm based on the length of the object IDs in the prerequisites and
ref advertisements.

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

bundle.h

@@ -15,6 +15,7 @@ struct ref_list {
 struct bundle_header {
 	struct ref_list prerequisites;
 	struct ref_list references;
+	const struct git_hash_algo *hash_algo;
 };
 
 int is_bundle(const char *path, int quiet);