Avoid unnecessary string copy

src/libutil/util.hh

@@ -712,7 +712,7 @@ inline std::string operator + (const std::string & s1, std::string_view s2)
 inline std::string operator + (std::string && s, std::string_view s2)
 {
     s.append(s2);
-    return s;
+    return std::move(s);
 }
 
 }