[clang-format] Don't flash an ugly cmd prompt in Vim on Windows

Reviewers: klimek

Differential Revision: http://llvm-reviews.chandlerc.com/D941

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183654 91177308-0d34-0410-b5e6-96231b3b80d8