refs/iterator: implement seeking for reftable iterators

refs/iterator: implement seeking for reftable iterators

Implement seeking of reftable iterators. As the low-level reftable
iterators already support seeking this change is straight-forward. Two
notes though:

  - We do not support seeking on reflog iterators. It is unclear what
    seeking would even look like in this context, as you typically would
    want to seek to a specific entry in the reflog for a specific ref.
    There is currently no use case for this, but if one arises in the
    future, we can still implement seeking at that later point.

  - We start to check whether `reftable_stack_init_ref_iterator()` is
    successful.

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