Fix issues with VTKmDetermineVersion.

The code had 2 issues:

1. used obsolete 'output' if Git executable was missing. There was a possibility that output variable was never set and used some garbage value from parent scope.

2. version pattern matching was too liberal and would match any long txt so long as it has some numbers in it that matched a typical version number.

see: paraview/paraview#18116 (closed)