Merge branch 'kn/update-ref-symref'

"git update-ref --stdin" learned to handle transactional updates of
symbolic-refs.

* kn/update-ref-symref:
  update-ref: add support for 'symref-update' command
  reftable: pick either 'oid' or 'target' for new updates
  update-ref: add support for 'symref-create' command
  update-ref: add support for 'symref-delete' command
  update-ref: add support for 'symref-verify' command
  refs: specify error for regular refs with `old_target`
  refs: create and use `ref_update_expects_existing_old_ref()`

builtin/receive-pack.c

@@ -1576,7 +1576,8 @@ static const char *update(struct command *cmd, struct shallow_info *si)
 		if (ref_transaction_delete(transaction,
 					   namespaced_name,
 					   old_oid,
-					   0, "push", &err)) {
+					   NULL, 0,
+					   "push", &err)) {
 			rp_error("%s", err.buf);
 			ret = "failed to delete";
 		} else {