Function laid_out 

pub fn laid_out(
    repo: &Path,
    from: &str,
    branch: Option<&str>,
    file: &str,
    at: Splice,
    what: &str,
) -> Result<(TempDir, PathBuf), Trouble>

A checkout of from with one class replaced, and nothing committed.

Shared by checking and forking on purpose: what gets measured has to be the same tree that gets committed, or a green light means nothing. branch cuts one; None leaves it detached, which is what a check wants — asking whether an edit survives should not litter a repository with the noes.