Remove the following comment in pullV2Tag:

// NOTE: not using TagService.Get, since it uses HEAD requests
// against the manifests endpoint, which are not supported by
// all registry versions.

This is actually not an issue, because TagService.Get does a fallback to
GET if HEAD fails. It has done this ever since TagService was added to
the distribution API, so this comment was probably based on an early
version of TagService before it was merged, or was always a
misunderstanding.

However, we continue to use ManifestService.Get instead because it
saves a round trip. The manifest can be retrieved directly instead of
resolving the digest first.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>