title: Split a repository into "sub-repositories" while retaining remote to a foreign repository
body: <p>My main dev project is a "total conversion" of a game named NAEV. My TC is named <strong>SoC</strong> (short for "Stars of Call"), which is set in a totally different universe from NAEV, but uses the game source as-is. With my limited C abilities, I sometimes lend a hand to <strong>NAEV</strong>. It's written in C and <strong>basically has two subfolders</strong> : <strong>src</strong> (C source) and <strong>dat</strong> (XML/Lua data).</p>
  
  <p>NAEV is stored on GitHub, in its repository named <strong>naev/naev</strong>, <strong>which I do not own</strong>. I have <strong>my own clone</strong> <strong>Mutos/naev</strong> on GitHub, which I keep in synch with naev/naev. For that, I use a local repository with <strong>origin</strong> pointing to Mutos/naev and <strong>upstream</strong> to naev/naev, like in <a href="https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/syncing-a-fork" rel="nofollow noreferrer">https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/syncing-a-fork</a>.</p>
  
  <p>So I develop SoC by trying to keep up the source with NAEV and focusing on the data, which describes the SoC universe. So the issue is, that SoC must have the <strong>same src</strong> folder than NAEV, but a <strong>totally different dat</strong> folder.</p>
  
  <p>For now, I synch by hand my SoC src folder with NAEV src folder. But that means I lose commits history and cannot gradually adapt my data according to the evolution of NAEV, commit by commit. So SoC is always trailing behind NAEV and each time I want to update SoC's src, I have a hard time adapting my data.</p>
  
  <p>What I want is setting up a third repository for SoC, named Mutos/SoC, <strong>whose src folder would follow NAEV src</strong>, while its dat folder would be totally independant from NAEV's dat folder.</p>
  
  <p>How can that be achieved ? Note that I'm not so skilled in GitHub, because I use it only occasionnally. So I'll need explanations to be worded for being understandable n00b-level ^-^</p>
  
  <p>Thanks for any hint !</p>