Move the ID-only case before checking for a digest
Move the ID-only case before checking for a digest

Digests are only valid for named references, so the two are
mutually exclusive and order does not quite matter.  To avoid a costly
store.Image() lookup for named references, check for "@:" first.

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

containers/image,containers/image