catalog: List repositories with no unique layers
catalog: List repositories with no unique layers

A repository need not contain any unique layers, if its images use only layers
mounted from other repositories. But, the catalog endpoint was looking for the
_layers directory to indicate that a directory was a repository.

Use the _manifests directory as the marker instead, since any repository with
revisions will contain a _manifests directory.

Signed-off-by: Adam Wolfe Gordon <655dd4039015d0b287f10e74a43e299906db51bc@digitalocean.com>

thaJeztah/distribution,sadlil/distribution,docker/distribution,docker/distribution,thaJeztah/distribution,sadlil/distribution