title: Detect merged branches from Git merge commit
body: <p>I want to know merged branches from merge commit.</p>
  
  <p>So, I decide to automatically write a merge message which contains merging branches when the merge happens by using a commit hook.</p>
  
  <p>However, this has a problem. I couldn't know which branches are merged when I just see a commit object.</p>
  
  <p>Is there a good way to know which branches are merged while just seeing a commit object?</p>