Update differ to support compressed archives

Differ is updated to set a label for the uncompressed hash
of compressed content. This allows compressed blobs to
be used and looked up for their uncompressed hashes.
Uses commit options to set labels.
Updates rootfs default to create compressed archives.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

rootfs/diff.go

@@ -49,5 +49,5 @@ func Diff(ctx context.Context, snapshotID, contentRef string, sn snapshot.Snapsh
 		defer sn.Remove(ctx, lowerKey)
 	}
 
-	return md.DiffMounts(ctx, lower, upper, ocispec.MediaTypeImageLayer, contentRef)
+	return md.DiffMounts(ctx, lower, upper, ocispec.MediaTypeImageLayerGzip, contentRef)
 }