Add kpod diff command

kpod diff reports on differences between two layers, specified as
layer IDs, containers, or images.  In the case of containers or
images, kpod diff produces a diff for the top layer

Signed-off-by: Ryan Cole <rcyoalne@gmail.com>

libkpod/layer/layer.go

@@ -0,0 +1,12 @@
+package layer
+
+import cstorage "github.com/containers/storage"
+
+// FullID gets the full id of a layer given a partial id or name
+func FullID(store cstorage.Store, id string) (string, error) {
+	layer, err := store.Layer(id)
+	if err != nil {
+		return "", err
+	}
+	return layer.ID, nil
+}