refs/reftable: introduce generic checks for refs

refs/reftable: introduce generic checks for refs

In a preceding commit we have extracted generic checks for both direct
and symbolic refs that apply for all backends. Wire up those checks for
the "reftable" backend.

Note that this is done by iterating through all refs manually with the
low-level reftable ref iterator. We explicitly don't want to use the
higher-level iterator that is exposed to users of the reftable backend
as that iterator may swallow for example broken refs.

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