Function start 

pub fn start(
    label: &'static str,
    total: Option<usize>,
    enabled: bool,
    quiet: bool,
) -> Guard

Install a thread-local progress reporter for the duration of the returned Guard. enabled = false installs no reporter, so report stays a cheap no-op — used when stderr isn’t interactive or --quiet was passed (see should_report).

total, when known ahead of time (the push side plans its pack before building it), renders as done/total; None (the fetch/pull side, where the object count isn’t known until each pack is downloaded) renders as a running count only — never a fabricated total.