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It's a space-saving mechanism; since forks are extremely common, sharing commit trees across all of GitHub makes the forking action very cheap.

So, OP has answered it and I missed it, Github shares commit hash between Forks and parent repo

Yeah, and it can lead to fun things like this: https://news.ycombinator.com/item?id=24882921

> However, I realized I could get around that issue by forking the actions/docker repo and pushing a commit to my fork (since GitHub shares commits between forks and parent repositories).