Function spawn_subtree_op 

pub fn spawn_subtree_op(workdir: PathBuf, op: SubtreeOp, line: &str) -> Effect

Run a subtree operation off the actor thread, reporting completion.

A notification rather than spawn_git’s fire-time echo: every subtree operation rewrites history or touches a remote, so “did it work” is the question, and an echo written before it starts cannot answer it.