Users still consistently get hung up on understanding the behavior of `arc diff`. Some specific things we could improve:
- My experience is that `arc which` is helpful to users, but users often don't know about it. `arc diff` should provide a clear path to discovery.
- It might make sense to provide an abbreviated version of the `arc which` output in the `arc diff` output?
- There's some very old, crufty default-base-revision code. We should remove it and move completely to `base` rules, which have proven powerful enough to express all desired behaviors.
- The default behavior of `arc diff` should probably be to prompt / walk through?
- Users still consistently have different expectations of what "review" means. We might be able to surface some of this documentation more prominently.
- When the `arc diff` is updating a revision, we could attempt to introduce an `arc:last` rule which uses the base commit of the previous revision. This is complicated because we need the base commit to figure out the revision, but in the explicit `--update` case we know it and the general case we might be able to figure it out (e.g., by looking a few commits back in history?)