title: vimdiff is not launching when I try to resolve a git merge conflict
body: <p>I am using git and trying to merge a branch which is causing a mergeconflict.<br>
  I have tried to configure <code>vimdiff</code> as the mergetool with</p>
  
  <pre><code>git config merge.tool vimdiff
  git config merge.conflictstyle diff3
  git config mergetool.prompt false
  </code></pre>
  
  <p>Here is what is happening</p>
  
  <pre class="lang-sh prettyprint-override"><code>$ git merge master
  CONFLICT (rename/delete): dev_tools+env_config/gnome-term_+_bash+tmux.sh deleted in HEAD and renamed in master.  
  Version master of dev_tools+env_config/gnome-term_+_bash+tmux.sh left in tree.
  Automatic merge failed; fix conflicts and then commit the result.
  
  $ git status
  On branch breaking_changes_test
  
  You have unmerged paths.
    (fix conflicts and run "git commit")
  
  Unmerged paths:
    (use "git add &lt;file&gt;..." to mark resolution)
  
          added by them:      dev_tools+env_config/gnome-term_+_bash+tmux.sh
  
  no changes added to commit (use "git add" and/or "git commit -a")
  
  $ git mergetool
  Merging:
  dev_tools+env_config/gnome-term_+_bash+tmux.sh
  
  Deleted merge conflict for 'dev_tools+env_config/gnome-term_+_bash+tmux.sh':
    {local}: deleted
    {remote}: created file
  Use (c)reated or (d)eleted file, or (a)bort?
  </code></pre>
  
  <p>When I ran <code>git mergetool</code>, I was expecting <code>vimdiff</code> to open showing the differences between the conflicted file.<br>
  How can I get <code>vimdiff</code> to launch?</p>