Method interaction_start_parent

Walk the interaction containing `rev` back along first-parents to its first delta, returning the rev the cursor would rest on after undoing this interaction, or `None` if that is the root (the earliest interaction, which is not undoable). Mirrors the boundary condition in [`undo`](Self::undo): stop when the parent is an `interaction_end` boundary or the root.

(&self, rev: Rev)