title: How to get branch name when running pipeline on tag?
body: <p>I run pipeline from a tag (let's say <code>v1.0.0</code>) on a branch (let's say <code>staging</code>).
  My output file is created with <code>${CI_PROJECT_NAME}-${CI_COMMIT_REF_NAME}.apk</code>. 
  The result is a file named <code>MyProject-v1.0.0.apk</code>.</p>
  
  <p>I wish add branch name in the output filename to get <code>MyProject-staging-v1.0.0.apk</code>.</p>
  
  <p><a href="https://docs.gitlab.com/ee/ci/variables/predefined_variables.html" rel="noreferrer">From the gitlab documentation</a>, i could use <code>CI_COMMIT_TAG</code> and <code>CI_COMMIT_BRANCH</code> like this <code>${CI_PROJECT_NAME}-${CI_COMMIT_BRANCH}-${CI_COMMIT_TAG}.apk</code>.
  But the documentation says:</p>
  
  <blockquote>
    <p>CI_COMMIT_BRANCH : The commit branch name. Present only when building
    branches.</p>
    
    <p>CI_COMMIT_TAG : The commit tag name. Present only when building tags.</p>
  </blockquote>
  
  <p>So how to get the branch name?</p>