How do I create a manual diff?
I have an existing codebase that I'd like to highlight particular portions of for demonstration purposes. We use Review Board, which allows a user to upload a diff and conveniently compare it against the repository in question. 

That given, is there a good way to create a diff manually so it will show the relevant portions as having been added? The only way that comes to mind is by deleting the relevant code and reversing the diff.
How do I create a manual diff?
===

I have an existing codebase that I'd like to highlight particular portions of for demonstration purposes. We use Review Board, which allows a user to upload a diff and conveniently compare it against the repository in question. 

That given, is there a good way to create a diff manually so it will show the relevant portions as having been added? The only way that comes to mind is by deleting the relevant code and reversing the diff.