title: Check if git branch has been incorporated into other branch without running `git merge`
body: <p>I have featurue branches that I squash before merging into the integration branch. Sometimes I have random feature branches lying around and I cannot remember if they were merged into the integration branch.</p>
  
  <p>The problem, is that if I run <code>git merge</code> to check, sometimes this will cause problems for feature branches that have <em>already</em> been merged + squashed into the integration branch. (If someone can explain exactly what this problem is, that would be nice, I assume it's because the squash and merge creates an extra commit).</p>
  
  <p>So how can I check if a branch has already been merged without running <code>git merge</code>?</p>