Saving an image via digested reference, ID or truncated ID doesn't store
the image reference in the archive. This also causes the save code to
not add the image's manifest to the index.json.
This commit explicitly adds the untagged manifests to the index.json if
no tagged manifests were added.
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
d131f00fff