git checkout: fix default head case

The "${new=$old}" syntax only works for an undefined 'new', not for an
empty one. I knew that. Really. I'm not stupid.

git-checkout-script

@@ -29,7 +29,7 @@ while [ "$#" != "0" ]; do
     esac
     i=$(($i+1))
 done
-: ${new=$old}
+[ -z "$new" ] && new=$old
 
 if [ "$force" ]
 then