refs: add peeled object ID to the `ref_update` struct

Certain reference backends {packed, reftable}, have the ability to also store the peeled object ID for a reference pointing to a tag object. This has the added benefit that during retrieval of such references, we also obtain the peeled object ID without having to use the ODB. To provide this functionality, each backend independently calls the ODB to obtain the peeled OID. To move this functionality to the generic layer, there must be support infrastructure to pass in a peeled OID for reference updates. Add a `peeled` field to the `ref_update` structure and modify `ref_transaction_add_update()` to receive and copy this object ID to the `ref_update` structure. Finally, modify `ref_transaction_update()` to peel tag objects and pass the peeled OID to `ref_transaction_add_update()`. Update all callers of these functions with the new function parameters. Callers which only add reflog updates, need to only pass in NULL, since for reflogs, we don't store peeled OIDs. Reference deletions also only need to pass in NULL. For others, pass along the peeled OID if available. In a following commit, we'll modify the backends to use this peeled OID instead of parsing it themselves. Signed-off-by: Karthik Nayak <karthik.188@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>