labels: allow restoring cache-map

E.g. with unexpected termination for mu it is possible that the
labels-cache (i.e., the one that is used for auto-completion) gets disconnected
from reality.

Add a --restore option to `mu label list` to restore the actual labels from the
labels seen in the store.

mu/mu-options.cc

@@ -528,6 +528,8 @@ sub_label(CLI::App& sub, Options& opts)
 
 	// list
 	[[maybe_unused]] auto list = sub.add_subcommand("list", "list labels in the store");
+	list->add_flag("--restore", opts.label.restore,
+			 "Restore the label-list from the labels in store");
 	add_muhome_option(*list, opts);
 
 	// export