This is something that Gerrit is really good at. The model is a little weird, because it tracks all pushes to a code review essentially as separate commits. But it makes the workflow really really easy for reviewers: you can diff any two versions of the proposed patch. Github has no equivalent that I've found. You can look at a subset of individual commits in the PR, but they're not diffed against each other, which makes it harder to check that the one or two things you were concerned about have been addressed.