title: How to ignore files or directories with clang-format 3.9
body: <p>I am currently using travis ci to check patches as they come into github and am trying to figure out if there is anyway for clang-format 3.9 (since travis ci will only support ubuntu 14.04 currently as latest) to ignore entire directories or files when scanning changes.</p>
  
  <p>My .travis.yml file:</p>
  
  <pre><code>language: c++
  sudo: required
  dist: trusty
  install:
  - sudo apt-get update
  - sudo apt-get install clang-format-3.9 python3
  - ./travisci/check_patch.py
  </code></pre>
  
  <p>My travisci/check_patch.py file:</p>
  
  <pre><code>#!/usr/bin/env python3
  
  from subprocess import Popen, PIPE, STDOUT
  
  # Run clang to check if code changes cause a diff output and return 1 if so.
  cmd = "git show origin/master..@ | clang-format-diff-3.9 -p 1 -style=file"
  diff = Popen(cmd, stdout=PIPE, shell=True).communicate()[0]
  if diff:
      print("Code formatting is not according to style guidelines. Read https://github.com/intel/IA-Hardware-Composer/wiki/Contributions#coding_style")
      exit(1)
  
  exit(0)
  </code></pre>