refs: introduce `.ref` field for the base iterator

refs: introduce `.ref` field for the base iterator

The base iterator has a couple of fields that tracks the name, target,
object ID and flags for the current reference. Due to this design we
have to create a new `struct reference` whenever we want to hand over
that reference to the callback function, which is tedious and not very
efficient.

Convert the structure to instead contain a `struct reference` as member.
This member is expected to be populated by the implementations of the
iterator and is handed over to the callback directly.

While at it, simplify `should_pack_ref()` to take a `struct reference`
directly instead of passing its respective fields.

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