title: Environment variables in .git/config
body: <p>I have a git repo created with the <code>--serparate-git-dir</code> option. I often use the same repo form different working trees by specifying <code>--git-dir</code> and <code>--work-tree</code> as arguments.</p>
  
  <p>I have two working trees I switch between frequently so I added a <code>.git</code> file in the secondary work tree pointing to the repository directory. However since the repository's <code>config</code> file points to the primary working tree, I still have to specify it explicitly, otherwise it uses the primary working tree.</p>
  
  <p>I tried setting the value of <code>worktree</code> to <code>$PWD</code> int the <code>.git/config</code> file but this causes the following error:
  <code>fatal: Could not chdir to '$PWD': No such file or directory</code></p>
  
  <p>Is there a way to make <code>worktree</code> dynamic?</p>