Fix VTK-m version for tarball in another git repository
A ParaView user noted that the determine version script (which is essentially the same for ParaView and VTK-m) incorrectly used the results of git describe when the source actually came from a tarball distribution that was placed in another git archive. (The SHA was incorrectly taken from the enclosing git project.) See the ParaView bug for more details on the report:
paraview/paraview#17761 (closed)
This fix simply checks to make sure the the source directory has the .git subdirectory expected in all git projects.