[CMake] Hack GetSVN.cmake to handle unusual terminals.

I got a report of a hang in git's helper functions trying to figure out
how to display results of "git svn info" when run inside ninja, even though
the result is immediately piped to grep. This seems to avoid that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190808 91177308-0d34-0410-b5e6-96231b3b80d8