Jenkinsfile: how can I checkout branch instead of refs/heads/branch?

Question

The following Jenkinsfile:

pipeline {
  agent any

  stages {
    stage( "1" ) {
      steps {
        script {
          def br = "demo_branch"
          def clone_url = "ssh://[email protected]:7999/spc/my_prj.git"
          def cred_id = "ssh_cred_id"
          def relative_dir = "my_prj"
          checkout([$class: 'GitSCM',
                    branches: [[name: br]],
                    doGenerateSubmoduleConfigurations: false,
                    extensions: [[$class: 'RelativeTargetDirectory',
                                  relativeTargetDir: relative_dir]],
                    submoduleCfg: [],
                    userRemoteConfigs: [[url: clone_url,
                                        credentialsId: cred_id]]])

        }
      }
    }
  }

  post {
    always {
      script {
        echo "the end"
      }
    }
  }
}

...results in the directory /home/jenkins/workspace/job/my_prj/ being created in my Jenkins server, and that directory contains my cloned git repo.

I would like to cd to this directory, run the command git rev-parse --abbrev-ref HEAD and get the value "demo_branch", but instead I get the value "HEAD". I am pretty certain that this is because the Jenkinsfile checkout command is checking out refs/heads/demo_branch instead of demo_branch. I.e.:

user@server:/home/user# cd /home/jenkins/workspace/job/my_prj/
user@server:/home/jenkins/workspace/job/my_prj# git rev-parse --abbrev-ref HEAD
HEAD
user@server:/home/jenkins/workspace/job/my_prj# git checkout demo_branch
Switched to branch 'demo_branch'
Your branch is up-to-date with 'origin/demo_branch'.
user@server:/home/jenkins/workspace/job/my_prj# git rev-parse --abbrev-ref HEAD
demo_branch

Is there a way I can get the Jenkins checkout command to check out demo_branch instead of refs/heads/demo_branch?