As the article notes, this is hard to do cleanly, depending on the exact semantics of what you want. And even with a full spec in hand, I don't think git's data model makes it particularly easy to compute this information.

One method for doing this manually is to do an interactive rebase and re-order all the relevant commits to the end. You basically create a new branch that has all the other commits first, and then you can diff between that point and the branch head. With the example commits from the post, you'd order them A B D F G C E H; then git diff G H would show just the changes from C E H.

This is a bit annoying in that it needs to use the working tree to apply all the commits in sequence, and you have to manually resolve any conflicts that arise, which gets more difficult the larger and more-intertwined the commits get. But maybe it's better than nothing...