Refuse to merge to bases other than `master`

We often create stacked PRs, using stgit or Graphite. This means we end up in positions where we have PRs which are dependent on other PRs.

- master
- feat-1
- feat-2 <-- feat-2 is based off of feat-1

I understand that with Change base before delete by champo · Pull Request #1023 · bors-ng/bors-ng · GitHub, bors handles the case where feat-1 gets merged before feat-2.

I'm talking about the case where feat-2 is merged before feat-1 is. In this case, bors will happily merge feat-2 into feat-1. I dislike this behavior -- it is never what I want with stacked PRs!

Is there any way to stop bors from performing the merge (ie fail the PR) if the PR's base branch is anything other than the default branch (ie master)?