Currently, we rely on the PR author to squash their commits. This results in an extra review cycle.

But often, they'll just end up squashing their commits into a single commit. In those cases, that extra cycle could be avoided by performing a squash automatically.

This feature would enable @bors r+ squash to automatically squash all commits in the PR into a single commit with a message composed of the PR title and body. The squash would be performed before roll-up if applicable, or otherwise before merge.