streaming: get rid of `the_repository`

streaming: get rid of `the_repository`

Subsequent commits will move the backend-specific logic of object
streaming into their respective subsystems. These subsystems have gotten
rid of `the_repository` already, but we still use it in two locations in
the streaming subsystem.

Prepare for the move by fixing those two cases. Converting the logic in
`open_istream_pack_non_delta()` is trivial as we already got the object
database as input.

But for `stream_blob_to_fd()` we have to add a new parameter to make it
accessible. So, as we already have to adjust all callers anyway, rename
the function to `odb_stream_blob_to_fd()` to indicate it's part of the
object subsystem.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>