reftable/stack: provide convenience functions to create iterators

reftable/stack: provide convenience functions to create iterators

There exist a bunch of call sites in the reftable backend that want to
create iterators for a reftable stack. This is rather convoluted right
now, where you always have to go via the merged table. And it is about
to become even more convoluted when we split up iterator initialization
and seeking in the next commit.

Introduce convenience functions that allow the caller to create an
iterator from a reftable stack directly without going through the merged
table. Adapt callers accordingly.

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