Pull from GitHub's provided merge ref.

As mentioned in a [previous discussion](https://github.com/vaneyckt/Jently/issues/2#issuecomment-12800074), GitHub provides a merged version of the pull request. Since we only test if the request can be merged without conflict, it's safe to use this reference instead.

In our case, developers make pull requests from their private fork of the main repo and the Jently user does not always have access to the fork, so checkout fails. Using the merged GitHub reference gets around this.