Strip reviewable boilerplate from PR message when creating merge commit

[edunham]

We get

<!-- Reviewable:start -->

---
This change is [<img src="https://reviewable.io/review_button.svg" height="35" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/saltfs/356)
<!-- Reviewable:end -->

in merge commits, like https://github.com/servo/saltfs/commit/39ef35482769763504853fc69484abf29de4d95b, on repos where we use Reviewable. It would be nice to omit the Reviewable boilerplate when creating the merge commit message.

[emilio]

Related: https://github.com/servo/servo/issues/11153