[giaour]

The central authority (here, Github or the repo owner) can rewrite history by rebasing and then force pushing. The only way you can detect that as a third party is by cloning the repo and periodically pulling to make sure the history of the main/master branch has not been altered.

If it has been altered, how can you prove that you didn't just rewrite history locally and then make a spurious claim? You would need others with local copies to vote on which history was legitimate.

[endisneigh]

if one were to actually implement what I'm describing, the transactions would need to be signed by the participating parties, so it wouldn't be possible to do what you're describing anyway.