ThirdParty: always use a subtree option for merging
ThirdParty: always use a subtree option for merging

keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,keithroe/vtkoptix