refs: use peeled tag values in reference backends

refs: use peeled tag values in reference backends

The reference backends peel tag objects when storing references to them.
This is to provide optimized reads which avoids hitting the odb. The
previous commits ensures that the peeled value is now propagated via the
generic layer. So modify the packed and reftable backend to directly use
this value instead of calling `peel_object()` independently.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>