Fix bash compatibility

The use of zsh `&!` syntax was causing bash to die in script parsing, even though this never gets executed. This adds `true` as a dummy command to get past parsing and allow both bash and zsh to execute the script.

gitprompt.sh

@@ -12,7 +12,9 @@ function async_run() {
 function async_run_zsh() {
   {
     eval "$@" &> /dev/null
-  }&!
+
+  # `true` is used here to allow bash to parse the script, as the zsh `&!` syntax will otherwise stop parsing prior to any execution.
+  }&! true
 }