The `resolve_rev` C method should really have been
`resolve_rev_optional` from the start - it is more obviously wrong
in Rust because the input parameter `allows_noent` controls
whether the returned `Option` can ever be `None`.
I debated adding this to the C bindings, and may still do so,
but eh it's faster to write + ship in Rust, and the future of ostree is
Rust anyways.