title: Why does git mergetool open 4 windows in vimdiff? (I'd expect 3)
body: <p>When I met a conflict, I tried to use <code>git-mergetool</code> to solve it. I typed:</p>
  
  <pre><code>&gt;git mergetool -t vimdiff
  </code></pre>
  
  <p>It opened <code>vimdiff</code> in 4-way, not 3-way. My split windows in vimdiff look like:</p>
  
  <pre><code>:ls
    1 #a   "Gemfile.lock"                 line 1
    2 %a   "Gemfile.lock.LOCAL.4828.lock" line 1
    3  a   "Gemfile.lock.BASE.4828.lock"  line 0
    4  a   "Gemfile.lock.REMOTE.4828.lock" line 0
  </code></pre>
  
  <p>What are they? I want a 3-way diff: target file, merge file and working file. How should I configure my git or vimdiff?</p>