t: allow skipping expected object ID in `ref-store update-ref`
We require the caller to pass both the old and new expected object ID to
our `test-tool ref-store update-ref` helper. When trying to update a
symbolic reference though it's impossible to specify the expected object
ID, which means that the test would instead have to force-update the
reference. This is currently impossible though.
Update the helper to optionally skip verification of the old object ID
in case the test passes in an empty old object ID as input.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>