title: git push a sub-directory into another main repo
body: <p>I have a big main project with several directories as <strong>subtrees</strong>.</p>
  
  <p>I want to <strong>push</strong> the changes in one specific subtree to its origin, which is a separate repository.</p>
  
  <p>The trouble seems to be, that the current <strong>subtree</strong> I want to push does not originally came from the repository I want to push into. It came from a different repo, via the <em>subtree guides</em> I found by googling. It just looks very similar.</p>
  
  <p>Big project layout, where <code>important_subtree</code> is the thing I am worried about.</p>
  
  <pre><code>~/devel/bigproject
    .git/
    some_subtree/
    other_subtree/
    important_subtree/
      abc.txt
      efg.txt   &lt;--- new version
      hij.txt
  </code></pre>
  
  <p>And the <code>important_subtree</code> is "strongly related" to that repo:</p>
  
  <pre><code>~/devel/important
     .git/
      abc.txt
      efg.txt   &lt;--- old version
      hij.txt
  </code></pre>
  
  <p>Now <code>~/devel/bigproject/important_subtree/efg.txt</code> has changed and I want to <strong>push the important_subtree</strong> onto the repo <code>~/devel/important</code>. So afterwards <code>~/devel/important/efg.txt</code> also has the changes.</p>
  
  <p>The only thing I managed to do is to push push <em>everything</em> in <em>bigproject</em> into <em>important</em>, which is obviously not what I want. Only the changes in the subtree should be pushed.</p>