Ref T10939. I don't think this is hugely important, but it doesn't clutter things up much and it's nice as a hint.

T4055 was the original request specifically asking for this. It wanted a separate bucket, but I think this use case isn't common/strong enough to justify that.

I would like to improve Differential's "X depends on Y" feature in the long term. We don't tend to use/need it much, but it could easily do a better and more automatic job of supporting review of a group of revisions.