[core] Make TDirectory::Append tolerant to identical objects.

When calling TDirectory::Append(obj, replace=true), and obj is already
in the directory, return without doing anything.
If obj has the same name as an existing object, but is physically
different, the usual warning is raised.

This will enable workflows such as:
   auto h = new TH1D(...);
   directory->Append(h, true);

After this change, the above lines work both with and without auto
registration to gDirectory. This is needed to prepare ROOT for working
without auto registration.

Author: hageboeck

core/base/src/TDirectory.cxx

@@ -205,6 +205,7 @@ void TDirectory::Append(TObject *obj, Bool_t replace /* = kFALSE */)
    if (replace && obj->GetName() && obj->GetName()[0]) {
       TObject *old;
       while (nullptr != (old = GetList()->FindObject(obj->GetName()))) {
+         if (old == obj) return;
          Warning("Append","Replacing existing %s: %s (Potential memory leak).",
                  obj->IsA()->GetName(),obj->GetName());
          ROOT::DirAutoAdd_t func = old->IsA()->GetDirectoryAutoAdd();