title: git subtree error "fatal: refusing to merge unrelated histories"
body: <p>I'm trying to figure out how 'git subtree' works. I've followed all directions on <a href="https://help.github.com/articles/about-git-subtree-merges/" rel="noreferrer" title="About subtree merges">this page</a>, but I always get an error trying to merge the subtree project in my own repo ('Step 2'): <code>fatal: refusing to merge unrelated histories</code>. </p>
  
  <p>I've read <a href="https://stackoverflow.com/questions/37937984/git-refusing-to-merge-unrelated-histories">this post</a>, and when I use the <code>--allow-unrelated-histories</code> option, it seems to work fine. However, I'm not sure whether I should use this...My impression is that the whole point of subtrees is to have unrelated histories within one repository, so it feels strange to have to add the option. Should I add it nevertheless, or am I doing something wrong?</p>
  
  <p>I'm using git v2.9.3 on osx 10.11.6</p>