Mmm.. the only way I know to get incremental diffs would be to apply each separately to pristine source trees, and then diff the two source trees, producing the desired incremental patch. Then you could apply the incremental to your (third) master tree on top of prior patches.

Or, use "patch --reverse" to remove the old patch (eg. v20), and then apply the new one (eg. v22) to your custom tree. That might be simpler for you.

I currently don't supply incremental patches for my stuff, which would otherwise be the best solution.

-ml