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-cmd-label.cc

@@ -113,11 +113,16 @@ label_clear(Mu::Store& store, const Options& opts)
 }
 
 static Result<void>
-label_list(const Mu::Store& store, const Options& opts)
+label_list(Mu::Store& store, const Options& opts)
 {
-	const auto label_map{store.label_map()};
+	if (opts.label.restore) {
+		if (!opts.quiet)
+			mu_println("labels: restoring list from store...");
+		if (const auto res = store.restore_label_map(); !res)
+			return res;
+	}
 
-	for (const auto& [label, n]: label_map)
+	for (const auto& [label, n]: store.label_map())
 		if (opts.verbose)
 			mu_println("{}: {}", label, n);
 		else

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

mu/mu-options.hh

@@ -209,7 +209,9 @@ struct Options {
 		bool dry_run{};				 /**< Merely print the messages that would be
 							  * labeled without doing so */
 		StringVec delta_labels;                  /**< labels to add (+) or remove (-) */
-		bool read_only{};                        /** do not require writable store */
+		bool read_only{};                        /**< do not require writable store */
+
+		bool restore{};                         /**< restore the labels list */
 
 		OptString file;                           /** file for import/export */