How to `git rebase --root` ?

Hello, I a trying to implement an interactive rebase to modify all commit in a repository, including the first one.
The git cli provides the --root option to do so but I am having trouble translating this to git2-rs.
Reading at libgit2 doc I am still not sure what argument is expected to perform a "root" rebase ?

repository.rebase(
  Some(&head), // Current branch
  None, // No upstream to get all reachable commits
  Some(&onto), // needs to be provided or we get the following error : "Error: invalid argument: 'upstream || onto'; class=Invalid (3)
" 
  None // No opt ? 
)?;

Doing so on a repository containing only the first commit, I am not able to reach it.
Any hint ?

Start with the repository.rebase entry point and the libgit2 git_rebase_init documentation linked in the issue, then inspect how upstream, onto, and options are represented by git2-rs. Reproduce the case with a repository containing only its first commit and document or test the valid behavior for an interactive root rebase.