Add the "upstream_*" and "not_run" statuses my mutating set_tasks.

def _populate_unknown_statuses(set_tasks):
    """
    Add the "upstream_*" and "not_run" statuses my mutating set_tasks.
    """
    visited = set()
    for task in set_tasks["still_pending_not_ext"]:
        _depth_first_search(set_tasks, task, visited)