// `head` is verified, thus we know that if the parent reference
// exists, its target actually matches the parent OID.
// The fact that the parent OID is a hash over all previous history
// makes it *incredibley unlikely* or rather *practically impossible*
// makes it *incredibly unlikely* or rather *practically impossible*
// that the same `/refs` blob re-appears in previous history.
// Thus, we can spare oureselves walking the history.
return Ok(head);