no need to take ownership of additional game for merge

src/bin/bitsy-merge.rs

@@ -12,7 +12,7 @@ fn main() {
     let mut game_a = Game::from(fs::read_to_string(game_a).unwrap()).unwrap();
     let     game_b = Game::from(fs::read_to_string(game_b).unwrap()).unwrap();
 
-    game_a.merge(game_b);
+    game_a.merge(&game_b);
 
     fs::write(output, game_a.to_string())
         .expect("Failed to write output file");