Function create_commit 

pub fn create_commit(
    repo: &Repository,
    ref_name: &str,
    tree_id: ObjectId,
    parent: Option<ObjectId>,
    message: &str,
) -> Result<ObjectId>

Create a commit on ref_name, optionally with a parent.

The ref is updated atomically — it must either not exist (when parent is None) or point to parent (when Some).