On the pull request page it would be nice to have a button that shows the URLs for cloning the SOURCE repository for the pull request. We often want to add the source as an additional remote in a clone to manually pull in the changes for testing prior to performing the merge.

As pull requests are part of the destination you already have the destination in the top right where it normally resides. Perhaps the source branch display just to the right of the pull request number/state display could be augmented to provide this.