Function run 

pub fn run(
    dir: &Path,
    tip: &str,
    tree: &str,
    check: &Check,
    limit: Duration,
    env: &[(&str, &str)],
) -> Result<Outcome, Box<dyn Error>>

Run sem check --base <tip> --json in dir, whose HEAD has tree tree (the tree to be published). A pass counts only if sem checked exactly that tree: untracked files in dir would make its verdict one about another tree.