The `reftable_table` interface is an internal implementation detail that
callers have no access to. Having direct access to this structure is
important though for a subsequent patch series that will implement
consistency checks for the reftable backend.
Move the structure into "reftable-table.h" so that it part of the public
interface.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>