Improving collaboration with forks

This is huge. With Hack Club (https://github.com/hackclub/hackclub) we get a fair amount of PRs from beginners that have one or two minor issues.

We eventually gave up trying to get beginners to fix their pull requests and would manually pull their work to our personal forks, fix it, and then open up new pull requests.

Very excited to see this fixed.

This is great. Occasionally I jump into helping with some PRs (for example https://github.com/adobe/brackets/pull/12251) and before the choices have been either making the changes and asking for the contributor to pull the changes in from some other branch, posting some `.diff`s or just pulling the changes in and pushing them in yourself. This will make all that much simple and best of all more collaborative.

I have been envisioning such a feature for a while. I can imagine some pitfalls, but I think that my teams will be working this into our approach pretty quickly.

For small tweaks, this is great. I have been wanting this for years. Oh yessss

Yes! Saves the PRs on PRs...