create_tarballs.bash fails to include vtk-m subdmodule in tarballs with git 2.28.0
vtk-m source files are not copied into tarballs.
The extracted source directory
VTK/ThirdParty/vtkm/vtkvtkm/vtk-m should exist. With git 2.28.0, it does not.
Note: I suspect the git version is at play here, but I do not know exactly which version the problem started with.