[hf] Try to avoid the list of cleanups in HistFactory.

HistFactory models can suffer from slowdowns of minutes during
destruction because the list of cleanups is visited over and over again.

Author: hageboeck

roofit/histfactory/src/Channel.cxx

@@ -464,8 +464,9 @@ TH1* RooStats::HistFactory::Channel::GetHistogram(std::string InputFile, std::st
     throw hf_exc();
   }
 
-
+  TDirectory::TContext ctx{nullptr}; // Suppress any kind of registration
   TH1 * ptr = static_cast<TH1 *>(hist->Clone());
+  ptr->SetBit(kMustCleanup, false); // Don't try to clean up this histogram
 
   if(!ptr){
     std::cerr << "Not all necessary info are set to access the input file. Check your config" << std::endl;