Adds a new rego builtin `attest.internals.reproducible_git_checksum`.
This is needed for verifying DOI provenance, see
https://github.com/docker/doi-image-policy/blob/main/slsa.md#doi-build-reproducible-git-checksum.
We use https://github.com/go-git/go-git for as much of this as possible,
but it doesn't support the actual archive operation, so we shell out to
`git` for that.
There is some similar unexported code in bashbrew, and we should
probably be using the same code in the build process as we are here.
I'll create a follow-up ticket to sort that out.
