Function scan_pinned 

pub async fn scan_pinned<S>(
    cwd: &Path,
    env: &HashMap<String, String, S>,
    prev: Option<&GitDiffSnapshot>,
    base: &str,
) -> GitDiffSnapshot
where
    S: BuildHasher,

A read pinned to one base revision: the branch-ahead layer is computed against base (a sha a refresh just resolved) rather than against the ref’s name, so this read and a content read taken beside it describe one commit even if the ref moves between them. Never fetches and never looks the PR up: the caller has already refreshed the base and is drawing it.