Print correct URL for pull requests in an other GitHub organization

[pothos]

For pull requests that were done in upstream repositories, the wrong PR link is printed because the flatcar-linux organization is used by default:
https://github.com/kinvolk/flatcar-build-scripts/blob/1adc2ce106c34465e14d9600f2295e151bca292d/coreos-overlay-diff.py#L81
Through some heuristic based on the merge commit we can try to resolve https://github.com/coreos/systemd/pull/XY or https://github.com/systemd-stable/systemd/pull/XY instead of https://github.com/flatcar-linux/systemd/pull/XY