refs/reftable: read references via `struct reftable_backend`

refs/reftable: read references via `struct reftable_backend`

Refactor `read_ref_without_reload()` to accept `struct reftable_backend`
as parameter instead of `struct reftable_stack`. Rename the function to
`reftable_backend_read_ref()` to clarify its scope and move it close to
other functions operating on `struct reftable_backend`.

This change allows us to implement an additional caching layer when
reading refs where we can reuse reftable iterators.

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