repl: fix overwrite for this._prompt

This patch https://github.com/joyent/node/commit/b5175003bca45e23bd20d9559096dafda3e0ad7c overwrites the this._prompt, it caused the bug:
```sh
$ node
> function foo() {
...(space + backtrace/delete)
> 
```
`this._prompt` has been overwritten by https://github.com/joyent/node/commit/b5175003bca45e23bd20d9559096dafda3e0ad7c#diff-b13d72249263845d8e8341db0426f9d3R401

/cc @indutny