Function start_rebase 

pub async fn start_rebase(repo_dir: &Path, base_branch: &str) -> RebaseOutcome

Start a rebase of the current branch onto the latest origin/<base_branch>.

If the rebase succeeds cleanly, returns Clean. If it hits conflicts, the working tree is left in a mid-rebase state and RebaseConflicts is returned with the list of conflicting files. The caller should resolve them and call rebase_continue, or abort_rebase to give up.