Update branch if it exists

Hello 👋

Thank you for this awesome library! I am building a utility to improve the developer experience for those with many repositories, much like your Octoherd. [Kopier](https://github.com/bjerkio/kopier) is designed with templating in mind with using Github Actions, like [in Tabetalt](https://github.com/tabetalt/workflows) where we distribute some standardized files.

I recently started a full refactor of Kopier, and testing out this library for the pushing of files and creating pull request. One feature that I've been asked for a lot internally is being able to update a pull request with new changes. Imagine that there's one repository where we have an open pull request, (maybe because we're not ready for the changes yet in that repo), but we've pushed yet another update to the _origin repository_ (in Tabetalt, it's named `workflows`). We don't want yet another pull request for those changes.

To track this, I opened bjerkio/kopier#31. I imagine we could take parts of this library, and implement this behaviour in Kopier. Or, better yet, maybe this is something that this library should support? Or is it supported?