Function stored_base 

async fn stored_base(
    cwd: &Path,
    default_branch: &str,
    decided_at: Instant,
) -> BaseRefresh

Answer the last completed check the store holds, or the base unchecked, pinned to the local tip read beside it.

A run that published while the tip was being read still counts, as long as it completed before that tip was read: its fetch wrote the ref the tip names, so the pair is one base. The store is re-read after the tip for that reason, and a measurement stamped after the tip is left for the next read rather than paired with a ref it does not describe.