refs/iterator: implement seeking for packed-ref iterators

refs/iterator: implement seeking for packed-ref iterators

Implement seeking of `packed-ref` iterators. The implementation is again
straight forward, except that we cannot continue to use the prefix
iterator as we would otherwise not be able to reseek the iterator
anymore in case one first asks for an empty and then for a non-empty
prefix. Instead, we open-code the logic to in `advance()`.

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