List command: `$post__in` is assigned twice

The value of the variable $post__in in the list subcommand is overridden after the first assignment.

```php
// Prepare the IDs for inclusion in the query.
$post__in = array_map( 'esc_sql', $ids );
$post__in = implode( ',', $ids );
```

https://github.com/trepmal/wp-revisions-cli/blob/master/inc/class-revisions-cli.php#L113-L116