Builder.untarPath(): always evaluate b.ContentDigester.Hash()
Builder.untarPath(): always evaluate b.ContentDigester.Hash()

Make sure we evaluate b.ContentDigester.Hash() every time the callback
returned by Builder.untarPath() is called, since it may be for a
different digest object than the previous time it was called, or
different even from when the callback was first retrieved.

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

Closes: #1955
Approved by: rhatdan

projectatomic/buildah,TomSweeneyRedHat/buildah,rhatdan/buildah,TomSweeneyRedHat/buildah,projectatomic/buildah,nalind/buildah-1,nalind/buildah,rhatdan/buildah,nalind/buildah-1,TomSweeneyRedHat/buildah,projectatomic/buildah,rhatdan/buildah,nalind/buildah-1,nalind/buildah