if there is a tar-data.json.gz present for an image layer, then use it
to create the tar archive, instead of the traditional graphdriver Diff.
Signed-off-by: Vincent Batts <vbatts@redhat.com>
Conflicts:
graph/graph.go
Preserve the entries from the tar archive for layers added to the graph.
With these entries and relative filesystem path, the tar archives can be
reassembled later.
Signed-off-by: Vincent Batts <vbatts@redhat.com>