streaming: explicitly pass packfile info when streaming a packed object

When streaming a packed object we first populate the stream with information about the pack that contains the object before calling `open_istream_pack_non_delta()`. This is done because we have already looked up both the pack and the object's offset, so it would be a waste of time to look up this information again. But the way this is done makes for a somewhat awkward calling interface, as the caller now needs to be aware of how exactly the function itself behaves. Refactor the code so that we instead explicitly pass the packfile info into `open_istream_pack_non_delta()`. This makes the calling convention explicit, but more importantly this allows us to refactor the function so that it becomes its responsibility to allocate the stream itself in a subsequent patch. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>