doctor: Tweak git-clean suggestion for empty dirs

git clean -f is not enough. Needs `-d` somtimes to
handle cases when we rename a dir.

Fixes Homebrew/homebrew#18544 and fixes Homebrew/homebrew#14224

Library/Homebrew/cmd/doctor.rb

@@ -924,7 +924,7 @@ def check_git_status
       If this a surprise to you, then you should stash these modifications.
       Stashing returns Homebrew to a pristine state but can be undone
       should you later need to do so for some reason.
-          cd #{HOMEBREW_REPOSITORY}/Library && git stash && git clean -f
+          cd #{HOMEBREW_REPOSITORY}/Library && git stash && git clean -d -f
       EOS
     end
   end