Retry failed integrations if master changes

[ruuda]

If integrating an approved pull request fails, either because of a build failure or a conflict, that that is dependent upon whatever master was for that try. If master changes, that should invalidate previous attempts to integrate a pull request.

[ruuda]

Likewise, leaving an LGTM comment is currently idempotent and only changes the approval status. That is nice in an ideal world, but in practice it might be useful to clear the integration status, and retry, even if the exact same commit is LGTM'ed again.