Add a functional argument to pass a digest to (ManifestService).GetByTag().
If the digest matches an empty manifest and nil error are returned.
See 1bc740b0d5 for server implementation.
Signed-off-by: Richard Scothern <richard.scothern@gmail.com>
Since the common package no longer exists, the testutil package is being moved
up to the root. Ideally, we don't have large omnibus packages, like testutil,
but we can fix that in another refactoring round.
Signed-off-by: Stephen J Day <stephen.day@docker.com>