track index.json with LFS: plain-git files mount truncated

.gitattributes

@@ -61,3 +61,4 @@ saved_model/**/* filter=lfs diff=lfs merge=lfs -text
 openvid-head/gops.u8 filter=lfs diff=lfs merge=lfs -text
 openvid-general/gops.u8 filter=lfs diff=lfs merge=lfs -text
 openvid_parts.json filter=lfs diff=lfs merge=lfs -text
+*/index.json filter=lfs diff=lfs merge=lfs -text