Function scan_within 

pub async fn scan_within<'a>(
    cwd: &'a Path,
    snapshot: &'a GitDiffSnapshot,
    base: Option<BaseState>,
    base_ref: Option<&str>,
    base_error: Option<&str>,
    budget: Duration,
) -> DiffContent

scan with its command budget spelled; the budget bounds the machine’s start of each child, not the read’s own work.

base_ref pins the branch range to one revision (a sha the drawing read resolved) instead of the snapshot’s ref name, so this read and the stats scan beside it cannot straddle two refs. base_error is why the refresh could not run, carried beside the state it left.