TFileMerger _actually_ delete directory only if we induced its creation/loading

This is the actual heart of 30fd4c7 which incorrect did the reverse
of its intent.

This fixes #8226

Author: pcanal

io/io/src/TFileMerger.cxx

@@ -557,10 +557,11 @@ Bool_t TFileMerger::MergeOne(TDirectory *target, TList *sourcelist, Int_t type,
    auto getDirectory = [&dirtodelete](TDirectory *parent, const char *name, const TString &pathname)
    {
       TDirectory *result = dynamic_cast<TDirectory*>(parent->GetList()->FindObject(name));
-      if (!result)
+      if (!result) {
          result = parent->GetDirectory(pathname);
-      else
-         dirtodelete.Add(result);
+         if (result && result != parent)
+            dirtodelete.Add(result);
+      }
 
       return result;
    };