I bet I wasn't the first person who thought this would have to be done by modifying actual file content — e.g. a dummy comment or something. That would clearly have been horrible, but the fact that git bases the checksum off the commit message is... surprising and fortunate, in this case!
It's a hash of everything that goes into a commit, including the commit message. The idea is that nothing that makes up a commit can change without changing the hash.
> It's a hash of everything that goes into a commit, including the commit message
... and, very notably, the hash of the parent commit. That is also part of the commit, which means that changing a parent commit would also imply changing the hashes of all later commits. This is sort of the whole point of git/version control.
This might be a stupid question, but does anyone call git history a blockchain, then? A centralized blockchain, without proof of work or proof of anything really of course, but still, it sounds like the basic blockchain idea is there