Function land 

pub fn land(
    repo: &Path,
    url: &str,
    tip: &str,
    branch: &str,
) -> Result<(), String>

Lands a green train: pushes tip to branch on the remote at url WITHOUT force, so git’s fast-forward rule is the race guard — if the branch moved since the train was built, the push is rejected and the caller should rebuild on the next round.

Errors

Push failures, including the non-fast-forward rejection.