Footer ends up in squashed commit messages

[jrtc27]

In llvm/llvm-project we only allow squash and merge, and configure the commit message to be the pull request's summary. This interacts poorly with gh stack's automatic insertion of the following footer into the PRs it creates:

---

<sub>Stack created with <a href="https://github.com/github/gh-stack">GitHub Stacks CLI</a> • <a href="https://gh.io/stacks-feedback">Give Feedback 💬</a></sub>

I naively assumed that, on squash and merge, something server-side would know to trim such footers, much like how git am knows to ignore any stat lines. However, there is no such detection, and instead this ends up in the commit message, which interacts particularly badly with #285, as then at least I would have been able to manually fix each one before they were committed.

Please either stop adding these footers or ensure that something will reliably strip them on commit when configured as above. Whilst resolving #285 would improve the situation, I would not consider that sufficiently appropriate UX.