[clang-format] Another attempt at python 3 compatibility

The entries in vim.current.buffer appear to be decoded strings, which
means that python3 won't allow invoking 'decode' on them. Keep the old
behavior when running under python2, but skip the error-inducing decode
step with python3..

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289308 91177308-0d34-0410-b5e6-96231b3b80d8