reftable/merged: expose functions to initialize iterators

reftable/merged: expose functions to initialize iterators

We do not expose any functions via our public headers that would allow a
caller to initialize a reftable iterator from a merged table. Instead,
they are expected to go via the generic `reftable_table` interface,
which is somewhat roundabout.

Implement two new functions to initialize iterators for ref and log
records to plug this gap.

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