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.

lib/mu-store-labels.cc

@@ -23,6 +23,25 @@
 
 using namespace Mu;
 
+Result<void>
+Mu::LabelsCache::restore(const Store& store)
+{
+	const auto res{store.run_query("")};
+	if (!res)
+		return Err(Error{Error::Code::Query,
+				"failed to run query: {}",
+				 *res.error().what()});
+	label_map_.clear();
+
+	for (auto&& item: *res) {
+		if (auto &&msg{item.message()}; msg) {
+			for (const auto& label: msg->labels())
+				increase(label);
+		}
+	}
+	return Ok();
+}
+
 namespace {
 constexpr std::string_view	path_key       = "path:";
 constexpr std::string_view	message_id_key = "message-id:";