upload-pack: convert to use `reference_get_peeled_oid()`

The `write_v0_ref()` callback is invoked from two callsites: - Once via `send_ref()` which is a callback passed to `for_each_namespaced_ref_1()` and `refs_head_ref_namespaced()`. - Once manually to announce capabilities. When sending references to the client we also send the peeled value of tags. As we don't have a `struct reference` available in the second case, we cannot easily peel by calling `reference_get_peeled_oid()`, but we instead have to depend on on global state via `peel_iterated_oid()`. We do have a reference available though in the first case, it's only the second case that keeps us from using `reference_get_peeled_oid()`. But that second case only announces capabilities anyway, so we're not really handling a reference at all here. Adapt that case to construct a reference manually and pass that to `write_v0_ref()`. Start to use `reference_get_peeled_oid()` now that we always have a `struct reference` available. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>