Make fish_config prompt color changes work more often

Fixes https://github.com/fish-shell/fish-shell/issues/505

output.cpp

@@ -136,7 +136,7 @@ static bool term256_support_is_native(void)
     return max_colors == 256;
 }
 
-bool output_get_supports_term256()
+bool output_get_supports_term256(void)
 {
     return support_term256;
 }

reader.cpp

@@ -815,6 +815,7 @@ void reader_repaint_if_needed()
 
     if (needs_reset)
     {
+        exec_prompt();
         s_reset(&data->screen, screen_reset_current_line_and_prompt);
         data->screen_reset_needed = false;
     }