"git stash push -u -- <pathspec>" gave an unnecessary and confusing
error message when there was no tracked files that match the
<pathspec>, which has been fixed.

* tg/stash-untracked-with-pathspec-fix:
  stash: drop superfluos pathspec parameter
  stash push -u: don't create empty stash
  stash push: avoid printing errors
  stash: fix nonsense pipeline