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

This fix simply checks to make sure the the source directory has the
.git subdirectory expected in all git projects.