title: Jenkins Pipeline checkout not building PR commits
body: <p>We are using the integrator workflow with github. I am taking over a partially set up Jenkins Pipeline build from a dev that left the team. Currently, whenever we create a PR against the master repo, Jenkins will trigger a build as defined in the Jenkinsfile included in the project. However, rather than building the commits in the PR Jenkins builds whatever is in master. I connected to the Jenkins host and navigated to the directory where it was creating the workspace for the PR. git log only shows commits that have already been merged to the repo. I verified that changes in the PR were not reflected in the current files in the workspace for the build job. I believe the checkout step in defined incorrectly, but I am unsure what components are incorrect/missing.</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-html lang-html prettyprint-override"><code>    stage('checkout') {
          checkout([$class: 'GitSCM',
                    branches: [[name: '*/master']],
                    doGenerateSubmoduleConfigurations: false,
                    extensions: [[$class: 'WipeWorkspace']],
                    submoduleCfg: [],
                    userRemoteConfigs: [[credentialsId: 'GithubCredentiuals',
                                         url: 'https://github-myrepo']]])
      }</code></pre>
  </div>
  </div>
  </p>
  
  <p>update: the build config includes a github organization, as well as the credentials referenced by the checkout config, and a Pipline Jenkinsfile project recognizer. I do not know what other items are relevant from this screen.</p>