Function index_under 

pub fn index_under(
    store: &mut Store,
    root: &Path,
    subdirs: &[String],
) -> Result<Stats, Box<dyn Error>>

Index root, or — when subdirs is non-empty — only those repo-relative subtrees of it. Unbounded: an explicit index is thorough. A whole-repo index also reconciles deletions; a subtree index is a seed (it gets those files in first) that leaves coverage warming, so normal warming continues over the rest of the repo through use.