refactor(_cd): reduce forks

Author: akinomyoga

bash_completion

@@ -2073,8 +2073,9 @@ _cd()
         return
     fi
 
-    local -r mark_dirs=$(_comp_readline_variable_on mark-directories && echo y)
-    local -r mark_symdirs=$(_comp_readline_variable_on mark-symlinked-directories && echo y)
+    local mark_dirs='' mark_symdirs=''
+    _comp_readline_variable_on mark-directories && mark_dirs=y
+    _comp_readline_variable_on mark-symlinked-directories && mark_symdirs=y
 
     # we have a CDPATH, so loop on its contents
     for i in ${CDPATH//:/$'\n'}; do