A git commit is composed of a number of headers (key: value fields) and a commit message.
There is a set of "standard headers" (tree, parent*, author, committer, encoding?), but then you can add more. In fact there's a set of semi-standard headers, as in headers git itself will add under some conditions: `gpgsig` and `gpgsig-sha256` if the commit is signed, and I think `mergetag` for signed tags. They are documented as part of the signature format but they're not "baseline" features: https://git-scm.com/docs/signature-format#_commit_signatures
But because of this, a git client should support arbitrary commit headers, and round-trip them even if it does not expose them.
I was trying to make something like this post a couple of years back and used custom headers, even made this repo with a few zeroes with no salt on the commit message and no shenanigans in the files: https://gitlab.com/pedroteosousa/teste/-/commit/000000005093...
I have this ugly code that finds the salt given the target hash, and another that actually creates the commit given the salt. Is not very useable, but I'll leave it here for anyone that finds it interesting: https://gitlab.com/pedroteosousa/useless-git-tools/-/tree/ma...
