refs/reftable: don't fail empty transactions in repo without HEAD

Under normal circumstances, it shouldn't ever happen that a repository has no HEAD reference. In fact, git-update-ref(1) would fail any request to delete the HEAD reference, and a newly initialized repository always pre-creates it, too. We have however changed git-clone(1) to partially initialize the refdb just up to the point where remote helpers can find the repository. With that change, we are going to run into a situation where repositories have no refs at all. Now there is a very particular edge case in this situation: when preparing an empty ref transacton, we end up returning whatever value `read_ref_without_reload()` returned to the caller. Under normal conditions this would be fine: "HEAD" should usually exist, and thus the function would return `0`. But if "HEAD" doesn't exist, the function returns a positive value which we end up returning to the caller. Fix this bug by resetting the return code to `0` and add a test. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>