repl: Mitigate vm #548 function redefinition issue

```js
node 🙈 ₹ git:(upstream ⚡

 repl-tmp-548) ./node
> function name() { return "node"; };
undefined
> name()
'node'
> function name() { return "nodejs"; };
undefined
> name()
'nodejs'
>
```

Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Lance Ball <lball@redhat.com>