Add blob state calculation to handle empty ambiguity
Add blob state calculation to handle empty ambiguity

The size and throwaway fields in the history can bother be
omitted, making the emptiness of a layer ambiguous. In these
cases download and check whether the content is empty.

Signed-off-by: Derek McGowan <e1c79a582b6629e6b39e9679f4bb964d25db4aa8@mcgstyle.net>

miaoyq/containerd,miaoyq/containerd,kunalkushwaha/containerd,crosbymichael/containerd,mikebrow/containerd,dmcgowan/containerd,vdemeester/containerd,crosbymichael/containerd,thaJeztah/containerd,hqhq/containerd,dmcgowan/containerd,chanezon/containerd,estesp/containerd,kunalkushwaha/containerd,mikebrow/containerd,AkihiroSuda/containerd,containerd/containerd,vdemeester/containerd,containerd/containerd,estesp/containerd,thaJeztah/containerd,rajasec/containerd