refs: add missing optimize implementation for debug ref backend

index 697adbd0dc..8dad7046cd 100644
@@ -124,6 +124,14 @@ static int debug_pack_refs(struct ref_store *ref_store, struct pack_refs_opts *o
 	return res;
 }
 
+static int debug_optimize(struct ref_store *ref_store, struct pack_refs_opts *opts)
+{
+	struct debug_ref_store *drefs = (struct debug_ref_store *)ref_store;
+	int res = drefs->refs->be->optimize(drefs->refs, opts);
+	trace_printf_key(&trace_refs, "optimize: %d\n", res);
+	return res;
+}
+
 static int debug_rename_ref(struct ref_store *ref_store, const char *oldref,
 			    const char *newref, const char *logmsg)
 {

@@ -444,6 +452,7 @@ struct ref_storage_be refs_be_debug = {
 	.transaction_abort = debug_transaction_abort,
 
 	.pack_refs = debug_pack_refs,
+	.optimize = debug_optimize,
 	.rename_ref = debug_rename_ref,
 	.copy_ref = debug_copy_ref,