All the solutions to syncing fork master to upstream master through GitHub web interface I've found suggest ways that will create a new commit in fork master. And that is by definition not syncing because they will differ after this. In theory it should be possible because whenever I use the Edit button on a file in upstream repo it creates a perfectly synced new branch in my fork in a moment. But simply creating a PR for getting upstream master in my fork gives three options:
Create a merge commit.: new commit -> Not syncing.
Squash and merge: new commit -> Not syncing.
Rebase and merge: unavailable even though I didn't touch the fork master, see screenshot below.
What conflicts? How is it possible? Is there really no way?
Ok, I'm only 99.99% sure I didn't change the master in my fork, how do I find out which commit is different using CLI on local clone if they do differ?
It's not a duplicate of How do you update a GitHub repo fork in the web interface? because that one doesn't require there to be no new commits.