Use the proper ref instead of always assuming it's a 'head' (might be a 'tag')

checkout.go

@@ -59,7 +59,7 @@ func main() {
 	}
 
 	err = w.Checkout(&git.CheckoutOptions{
-		Branch: plumbing.ReferenceName("refs/heads/" + refName),
+		Branch: plumbing.ReferenceName(ref),
 	})
 	if err != nil {
 		log.Fatal(err)