Don't require two arguments in 'oci-storage diff'
Don't require two arguments in 'oci-storage diff'

We can figure out the parent layer ID by ourselves, so don't just return
an error if we weren't explicitly told which one to use as a reference.

Signed-off-by: Nalin Dahyabhai <f562fcb2a0b898bf0be1afc867ea92325d8f825f@redhat.com>

rhatdan/storage,rhatdan/storage,containers/storage,containers/storage,containers/storage,rhatdan/storage