title: what is git branch name in jenkins pipeline when invoked by gitlab webhook
body: <p>I can invoke a simple Jenkins pipeline from gitlab merge requests using a webhook. Now I'd like to know what is the source branch to make the checkout against it. Example: if I push code to develop branch, in my pipeline script i'd checkout develop branch. Thanks.</p>
  
  <pre><code>node {
    stage('Build') {
      def mybranch = '?' // get branch name from gitlab webhook
      git branch: mybranch,
      credentialsId: 'mycredential',
      url: 'myurl'
      // ...
    }
  }
  </code></pre>