human_readable_duration, Terminal::appendstring: Pass strings by reference

Found by cppcheck.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

src/terminal/terminaldisplay.h

@@ -58,7 +58,7 @@ namespace Terminal {
     }
 
     void append( const char * s ) { str.append( s ); }
-    void appendstring( const std::string s ) { str.append( s ); }
+    void appendstring( const std::string &s ) { str.append( s ); }
 
     void append_silent_move( int y, int x );
   };