packed_object_info(): use object_id for returning delta base

If a caller sets the object_info.delta_base_sha1 to a non-NULL pointer,
we'll write the oid of the object's delta base to it. But we can
increase our type safety by switching this to a real object_id struct.
All of our callers are just pointing into the hash member of an
object_id anyway, so there's no inconvenience.

Note that we do still keep it as a pointer-to-struct, because the NULL
sentinel value tells us whether the caller is even interested in the
information.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object-store.h

@@ -300,7 +300,7 @@ struct object_info {
 	enum object_type *typep;
 	unsigned long *sizep;
 	off_t *disk_sizep;
-	unsigned char *delta_base_sha1;
+	struct object_id *delta_base_oid;
 	struct strbuf *type_name;
 	void **contentp;