Pull requests: check both base & head for being a fork

[gr2m]

What happened?

See Mattlk13/p5.js#92. The linked pull request is unusual: Mattlk13/p5.js#92. The pull request head is the original repository, while base is the fork. It's a PR to keep a fork up-to-date.

What did you expect to happen?

There should not be an error.

What the problem might be

twitter-together currently only checks if head is a fork, but not base.

I assume that if we check for both, the problem should go away