initial pass at async 'git status' in prompt

- while it works well for me in my testing, this is meant
  more as a basis of discussion for 1) whether it makes sense
  for this to be in posh-git and 2) if so, what it should
  look like.  This version intentionally doesn't refactor out
  common code with the synchronous path since the goal is
  validating/rejecting the idea instead of the particular impl
change placeholder text to be shorter