Reduce the number of call to 'flush' in case of incremental merger

git-svn-id: http://root.cern.ch/svn/root/trunk@41428 27541ba8-7e3a-0410-8455-c3a389f83636

io/io/src/TFileMerger.cxx

@@ -709,7 +709,11 @@ Bool_t TFileMerger::MergeRecursive(TDirectory *target, TList *sourcelist, Int_t
       }
    }
    // save modifications to the target directory.
-   target->SaveSelf(kTRUE);
+   if (! type&kIncremental) {
+      // In case of incremental build, we will call Write on the top directory/file, so we do not need
+      // to call SaveSelf explicilty.
+      target->SaveSelf(kTRUE);
+   }
    return status;
 }