Add --xh option for xhprof profiling

In order to use this, you have to have the xhprof php extension enabled and have configured two other options: xh-path and xh-link
Add caching to drush_get_commands()

We were calling drush_get_commands() in topic_drush_help_alter() and thats expensive. Noticed  during site-alias command (for example) which needs to call topic_drush_help_alter() once for each loaded alias.
Fix reference in topic_drush_help_alter() param