Make Hoff handle external PRs correctly

[maartenberg]

I've just noticed an issue with merging external PRs that I think we haven't seen before.
When merging channable/vaultenv#113 and channable/vaultenv#115 Hoff did succeed in rebasing the PR, but after Hoff pushed the change to master the PR remained open in Github.

I think this happened because the PRs came from an external repository: if the PR isn't current with the repository's master branch Hoff will rebase the PR and force-push to the branch the PR came from.
Hoff doesn't seem to detect that the branch to push to is in another repository, and it instead seems to have pushed to the original repository (so channable/vaultenv instead of ruuda/vaultenv).

To fix this I think we should alter Hoff to push to the downstream repository instead.
This should be possible if we require the Allow edits from maintainers to be set on external PRs.

[ruuda]

Aaah of course, it will force-push, but to the wrong branch! That’s going to be interesting, probably the webhook for pull request updates includes a repository, then Hoff can add that as remote, and try to force-push to it.