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

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>