unpack/repack: save the --from argument

[cyphar]

When we do an umoci unpack we currently only keep track of the manifest hash that we extracted. We should also save (in umoci.state or something) the name of the reference that we dereferenced in order to get the extracted rootfs. This will allow us to remove the tedious --from argument in umoci repack.

The only real question to ask is whether we should make it possible for someone to forcefully create a rootfs that we cannot be sure is valid (namely specify --from and --mtree manually).

[cyphar]

This is being worked on in https://github.com/cyphar/umoci/tree/store-unpack-arguments.