zsh: complete qguard with +/- guard prefixes

contrib/zsh_completion

@@ -519,6 +519,7 b' typeset -A _hg_cmd_globals'
 _hg_qguards() {
   typeset -a guards
   local guard
+  compset -P "+|-"
   _hg_cmd qselect -s | while read guard
   do
     guards+=(${guard#(+|-)})