Function classify_rev_parse 

fn classify_rev_parse(
    output: GitOutput,
    presence: RepoPresence,
) -> Result<String, RepoGate>

Classify git rev-parse --abbrev-ref HEAD output into either the branch string to proceed with, or the terminal repo_gate. Empty stdout is a clean “not a repo” signal (git ran and reported it); Oversize means the scan subprocess itself misbehaved.

presence resolves the non-zero-exit case, which is ambiguous on its own: git exits 128 both outside a repo and when it fails inside one, so only the narrower repo-existence question separates a legitimate non-repo from a sick scanner.