Function checkout 

pub fn checkout(
    repo: &Repository,
    rev: &str,
    paths: &[PathBuf],
    force: bool,
) -> Result<CheckoutOutcome>

Restore the working tree to a revision without moving the current branch (a “restore-like” checkout). HEAD stays attached to its branch.

If paths is non-empty, only those files are restored from the revision (and nothing is deleted); each must exist in that revision.