[TreeProcMT] Actually fix TTrees in subdirectories
[TreeProcMT] Actually fix TTrees in subdirectories

ROOT-9563 was still broken for TTrees (the test was not testing things
properly).

root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,karies/root,olifre/root,root-mirror/root