Why not just do that locally?
Github lets you see the diff for a single commit on their site. This makes sense, because you can use this information to take action on their site, by either approving or denying the pull request.
The only use case for commit-commit diffs that I can think of, though, is if you plan on manually merging them and resolving conflicts, which you cannot do on Github (to my knowledge). There exist many graphical and command-line utilities that you can run on your machine, the same place you will be making those changes.
To me, Github's two jobs are to host reposiitories, and to make it easy to find and view the repositories of others. They do both of these jobs very well, and this map plugin makes Github better at the second in at least some cases. Once it comes time to actually start changing things, though, I get off of Github, and work on the local copy I have with the tools on my machine. Replicating this functionality isn't, to me, what Github is for.
As an aside, if there is a way to manually construct a url, there's a shortcut to getting Github to add it to the UI. A few months ago, someone made a JavaScript bookmarklet to add a search bar to any repository, since the UI for this functionality was so clunky. Because of its popularity, Github added a proper repository-specific search bar.