[RFC] Some simple fixes to GetGitRevisionDescription.cmake.

Unlike shell, we don't need to quote when expanding variables--CMake handles that for us.  So the extra quoting has been removed.  I did double check this to make sure it works correctly, and it does, though there are other things in our build that do not.

The actual fix here was to stop trying to manipulate `${out}` after the command failed.  I believe the other clause should've been in an else.  Instead of simply adding that, I re-arranged the if to check for success, rather than failure.