Describe the enhancement and the motivation for it.
For a large feature it's common to split the changes up into multiple smaller more focused changes. It's also common to keep working on those changes while a change is in review. It would speed up development when using ReviewBoard if you could specify a base review diff for another diff.
So for instance if I could say r/10 is based on r/9 which is also in review and ReviewBoard would only show me the differences between r/9 and r/10 then both related changes could be under development and review in parallel.
