bisect: consistently write BISECT_EXPECTED_REV via the refdb

bisect: consistently write BISECT_EXPECTED_REV via the refdb

We're inconsistently writing BISECT_EXPECTED_REV both via the filesystem
and via the refdb, which violates the newly established rules for how
special refs must be treated. This works alright in practice with the
reffiles reference backend, but will cause bugs once we gain additional
backends.

Fix this issue and consistently write BISECT_EXPECTED_REV via the refdb
so that it is no longer a special ref.

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