Function changed_since 

pub fn changed_since(
    root: &Path,
    path: &str,
    start: Option<u32>,
    end: Option<u32>,
    content_sha256: &str,
) -> Option<bool>

Whether path lines start–end no longer say what a read returned (content_sha256): read again with the same tool, so masking and the read cap are the same. None when the file cannot be read now.