Make GetSVN.cmake do its VCS queries with native CMake code.
This lets the queries work on Windows as well as Linux.

This does mean make and cmake aren't using the same scripts to do the
queries (again), but at least GetSVN.cmake understands git and git-svn
as well as svn now.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223425 91177308-0d34-0410-b5e6-96231b3b80d8