Fix error if index.php is in the installation url.

If when installing, the person used http://domain/index.php, the `$blog_url` in the `config.php` file will contain http://domain/index.php. 

This basically checks the last part of the `REQUEST_URI` and compares it to the last part of `__FILE__`. If they're the same, it is removed from the end of the `REQUEST_URI`.

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The only problem I see with this is if someone installs into a folder named `index.php` or under an alias with that name.