fish_prompt/_is_git_dirty: Initialize `untracked` with the empty list

Fix #4

functions/fish_prompt.fish

@@ -9,7 +9,7 @@ end
 
 function _is_git_dirty
   set -l show_untracked (git config --bool bash.showUntrackedFiles)
-  set untracked ''
+  set -l untracked
   if [ "$theme_display_git_untracked" = 'no' -o "$show_untracked" = 'false' ]
     set untracked '--untracked-files=no'
   end