title: Why doesn't git diff show anything for unmerged paths
body: <p>When merging two branches, many times the merge fails, and I have to resolve the diffs individually.</p>
  <p>Some files merge successfully, and I can see the diff:</p>
  <pre><code>git diff --staged &lt;merged-file&gt;
  </code></pre>
  <p>The problem is that for unmerged paths, when I try to check the diffs introduced, I see nothing:</p>
  <pre><code>git diff &lt;unmerged-file&gt;
  </code></pre>
  <p>But when I try to diff it against the remote master branch, then the diffs appear:</p>
  <pre><code>git diff origin/master &lt;unmerged-file&gt;
  </code></pre>
  <p>Why is this happening? Is the merge operation updating my local HEAD or something?<br />
  Moreover, when i use <code>git add</code> to mark resolution, they never go into the staging area -- is that the reason why <code>git diff </code> is not showing anything?</p>