This kind of annoys me a little. Basically I'll be working on a feature branch and I've submitted a diff, but I'm still waiting for it to be accepted. I want to start the next piece of work, so I `arc feature` off the feature branch (thus it's master -> first_feature_being_reviewed -> second_feature).
When the review is accepted and landed, I then have to do `git rebase --onto` to move the second feature branch on top of master. Ideally `arc land` would detect any derived branches and rebase them automatically, but I'm guessing this could be quite difficult to do properly (especially if there's 3 or 4 feature branches).