title: How to check is repo is in "dirty" status asynchronously? (For prompt)
body: <p>I have an usual displaying of info from GIT like current branch name, and is repo is in "dirty" state within prompt. The problem is that some larger repos take a good few seconds to calculate the "dirty" state.</p>
  
  <p>Is there a way to issue the dirty check asynchronously, to not hold down the responsiveness of shell?
  PS I'am using <code>fish 2.0</code> shell.</p>