Function refresh_once 

pub async fn refresh_once<F, Fut>(
    cache: &RwLock<GitCache>,
    folders: Vec<(String, String)>,
    now_ms: u64,
    timeout: Duration,
    probe: F,
)
where
    F: Fn(String) -> Fut,
    Fut: Future<Output = Option<GitFacts>>,

One refresh pass: probe each due (block, folder) in folders, every distinct folder once and at most MAX_CONCURRENT_PROBES at a time, each abandoned after timeout, and store the results stamped now_ms.