title: Git: branches differ, but no files to merge
body: <p>I work in a feature branch of a git repo. Meanwhile, the <code>master</code> branch changed:</p>
  
  <pre><code>my_feature$ git diff | wc -l
       388
  my_feature$ git diff master | wc -l
       388
  my_feature$
  </code></pre>
  
  <p>When I want to launch <code>mergtool</code>, I get the following:</p>
  
  <pre><code>my_feature$ git mergetool master
  No files need merging
  my_feature$ git mergetool
  No files need merging
  my_feature$
  </code></pre>
  
  <p>However, when I do <code>git merge master</code>, I recieve the following:</p>
  
  <pre><code>my_featurei$ git merge master
  Auto-merging some_dir/some_file.php
  CONFLICT (content): Merge conflict in Auto-merging some_dir/some_file.php
  Automatic merge failed; fix conflicts and then commit the result.
  </code></pre>
  
  <p>Why is this happening and now to fix this?</p>