I’m somewhat familiar with how git works. In my understanding, a commit is just a blob combining the commit information and a tree blob, hashing them together to create a commit id.
This design doesn’t preclude the usage of additional information in the commit blob that isn’t used to compute the hash.
(Think for example how file access times do not affect its hash)
Git is a content-addressed object store, the address of any stored object is the hash of the object itself. So you actually can't stuff extra data into an object and not change its ID; this auxiliary data would need to go in a separate store indexed by object ID or a similar solution. The reason why file access times don't affect git hashes is because git does not store them.
