Function layer_from_diffid 

fn layer_from_diffid<'a>(
    manifest: &'a ImageManifest,
    config: &ImageConfiguration,
    diffid: &str,
) -> Result<&'a Descriptor>

Given a target diffid, return its corresponding layer. In our current model, we require a 1-to-1 mapping between the two up until the ostree level. For a bit more information on this, see https://github.com/opencontainers/image-spec/blob/main/config.md