`conflict-marker` should exclude merged/closed PRs #2582

Example: https://github.com/sindresorhus/refined-github/pulls?q=is%3Apr+sort%3Aupdated-desc+is%3Aclosed

<img width="611" alt="Screenshot 2019-12-01 at 20 14 05" src="https://user-images.githubusercontent.com/1402241/69914451-299c6300-1477-11ea-8f2c-c26b2f0caa96.png">

It's a closed PR, conflicts are irrelevant