fmt(): Pass arguments by reference rather than by value

src/libutil/logging.hh

@@ -158,7 +158,7 @@ extern Verbosity verbosity; /* suppress msgs > this */
 #define vomit(args...) printMsg(lvlVomit, args)
 
 template<typename... Args>
-inline void warn(const std::string & fs, Args... args)
+inline void warn(const std::string & fs, const Args & ... args)
 {
     boost::format f(fs);
     nop{boost::io::detail::feed(f, args)...};

src/libutil/types.hh

@@ -67,7 +67,7 @@ inline std::string fmt(const FormatOrString & fs)
 }
 
 template<typename... Args>
-inline std::string fmt(const std::string & fs, Args... args)
+inline std::string fmt(const std::string & fs, const Args & ... args)
 {
     boost::format f(fs);
     f.exceptions(boost::io::all_error_bits ^ boost::io::too_many_args_bit);