title: Fail a merge or build if a particular file has changed?
body: <p>On VSTS, we have some files we want to protect on <strong><em>certain</em></strong> branches. How can one fail a merge/build if a particular file has changed?</p>
  
  <p>First prize is to configure this on the build server, which in this case is VisualStudio.com (VSTS / GIT).</p>
  
  <p>Scenario: we have various release branches v1, v2, v3. We want to protect the packages.json file to prevent anyone updating Nuget packages on these branches. So if the package.json file has changed on a pull request into "v3", don't allow the merge.</p>