Default to own fork on "Create pull request" if it exists and not a maintainer

[altrisi]

Description

Currently, when you click the Create pull request button in the pulls tab, it'll default to comparing across branches on the same repository. If you want to use a branch from your fork, you have to make multiple clicks, search the fork list, wait for a reload and then choose the branch.

If you don't work on the repository, you most likely want to create a pull request with a branch from your fork instead of between branches on the same repository. Refined GitHub could default to the fork in that case.

Example URLs

https://github.com/refined-github/refined-github/pulls