reftable: pick either 'oid' or 'target' for new updates

When creating a reference transaction update, we can provide the old/new
oid/target for the update. We have checks in place to ensure that for
each old/new, either oid or target is set and not both.

In the reftable backend, when dealing with updates without the
`REF_NO_DEREF` flag, we don't selectively propagate data as needed.
Since there are no active users of the path, this is not caught. As we
want to introduce the 'symref-update' command in the upcoming commit,
which would use this flow, correct it.

Helped-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak

refs/reftable-backend.c

@@ -898,8 +898,9 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 				 */
 				new_update = ref_transaction_add_update(
 					transaction, referent.buf, new_flags,
-					&u->new_oid, &u->old_oid, u->new_target,
-					u->old_target, u->msg);
+					u->new_target ? NULL : &u->new_oid,
+					u->old_target ? NULL : &u->old_oid,
+					u->new_target, u->old_target, u->msg);
 
 				new_update->parent_update = u;