Only compute {un,}compressedDigester.Digest() once
Only compute {un,}compressedDigester.Digest() once

It is a tiny bit expensive, but most importantly
this moves the uses of {un,}compressedDigester so that
we can later make them optional.

Should not change behavior.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/storage,containers/storage,containers/storage