When opening the read stream for a specific object the caller is also expected to pass in a pointer to the object type. This type is passed down via multiple levels and will eventually be populated with the type of the looked-up object. The way we propagate down the pointer though is somewhat non-obvious. While `istream_source()` still expects the pointer and looks it up via `odb_read_object_info_extended()`, we also pass it down even further into the format-specific callbacks that perform another lookup. This is quite confusing overall. Refactor the code so that the responsibility to populate the object type rests solely with the format-specific callbacks. This will allow us to drop the call to `odb_read_object_info_extended()` in `istream_source()` entirely in a subsequent patch. Furthermore, instead of propagating the type via an in-pointer, we now propagate the type via a new field in the object stream. It already has a `size` field, so it's only natural to have a second field that contains the object type. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>