old_contents: new GitGraphWrapper({orientation: 'vertical-reverse'})
      .branch('master')
      .checkout('master')
      .commit()
      .commit()
      .tag('v1.0')
      .branch('develop')
      .checkout('develop')
      .commit('Hello')
      .tag('tag2')
      .commit()
      .checkout('master')
      .commit()
      .orphanBranch('orphan')
      .checkout('orphan')
      .commit()
      .commit()
      .checkout('develop')
      .commit()
      .checkout('master')
      .merge('develop', {message: 'merge from develop'})
      .merge('orphan')
  ;
  
new_contents: new GitGraphWrapper({orientation: 'vertical-reverse'})
      // create 'master' branch and checkout it.
      .branch('master')
      .checkout('master')
      .commit()
      .commit()
      .tag('v1.0')
  
      // create 'develop' branch and checkout it.
      .branch('develop')
      .checkout('develop')
      .commit('Hello')
      .tag('tag2')
      .commit()
  
      // checkout master
      .checkout('master')
      .commit()
  
      // create orphan branch 'orphan' and checkout it.
      .orphanBranch('orphan')
      .checkout('orphan')
      .commit()
      .commit()
  
      // checkout 'develop'
      .checkout('develop')
      .commit()
  
      // checkout 'master' and merge 'develop' and 'orphan' branches.
      .checkout('master')
      .merge('develop', {message: 'merge from develop'})
      .merge('orphan')
  ;
  
subject: add comment