How to download a single commit-diff from GitHub?

OK, found the answer myself.

Adding .patch (or .diff) to the commit-URL will give a nice patch:

https://github.com/foo/bar/commit/${SHA}.patch

Thanks to Ten Things You Didn't Know Git And GitHub Could Do...

I found I had to add a / at the end, i.e.

https://github.com/foo/bar/commit/${SHA}.patch/