Async PROMPT_COMMAND

Speed up prompt by updating in the background. Might lead to more
~/.z.XXXX litter and less accurate counts - let's see.

z.sh

@@ -243,7 +243,7 @@ elif type complete >/dev/null 2>&1; then
     [ "$_Z_NO_PROMPT_COMMAND" ] || {
         # populate directory list. avoid clobbering other PROMPT_COMMANDs.
         grep "_z --add" <<< "$PROMPT_COMMAND" >/dev/null || {
-            PROMPT_COMMAND="$PROMPT_COMMAND"$'\n''_z --add "$(command pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null;'
+            PROMPT_COMMAND="$PROMPT_COMMAND"$'\n''(_z --add "$(command pwd '$_Z_RESOLVE_SYMLINKS' 2>/dev/null)" 2>/dev/null &);'
         }
     }
 fi