Bug: crashes when PR was merged on GH, but rebasing to `main` wasn't done

[cheshire]

If the PR was merged using GH UI, and the user tries to run stack-pr export without doing rebase manually first, they'll get a crash. Ideally a tool should be smart enough to suggest running rebase to the user in the error message instead of throwing a stack trace.

[thamenato]

Also after doing the rebase to fix the error, when running stack-pr submit it will remove the hyperlink reference to the PR that has been merged which is fine but would be nice to keep the reference even after rebasing.