builtin/show-ref: convert to use `reference_get_peeled_oid()`

builtin/show-ref: convert to use `reference_get_peeled_oid()`

The git-show-ref(1) command has multiple different modes:

  - It knows to show all references matching a pattern.

  - It knows to list all references that are an exact match to whatever
    the user has provided.

  - It knows to check for reference existence.

The first two commands use mostly the same infrastructure to print the
references via `show_one()`. But while the former mode uses a proper
iterator and thus has a `struct reference` available in its context, the
latter calls `refs_read_ref()` and thus doesn't. Consequently, we cannot
easily use `reference_get_peeled_oid()` to print the peeled value.

Adapt the code so that we manually construct a `struct reference` when
verifying refs. We wouldn't ever have the peeled value available anyway
as we're not using an iterator here, so we can simply plug in the values
we _do_ have.

With this change we now have a `struct reference` available at both
callsites of `show_one()` and can thus pass it, which allows us to use
`reference_get_peeled_oid()` instead of `peel_iterated_oid()`.

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