ThirdParty: support 2.9.0's new merge restrictions
ThirdParty: support 2.9.0's new merge restrictions

This flag is required for newer git. Declare that we Know What We're
Doing™ (but only if necessary).

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