Incorrect detection of backup dir inside the Drupal root

When updating a module, the function drush_prepare_backup_dir (in drush/includes/drush.inc) is called. It tries to detect if the backup directory is inside the Drupal root. Since the Drupal root doesn't end with a slash, you can get false positives.

To reproduce:
1) Drupal root - /var/www/drupal
2) Drush backup-dir - /var/www/drupal_backup

With this setup "strpos($backup_parent, $drupal_root)" will return 0 since $backup_parent is /var/www/drupal_backup/databasename and $drupal_root is /var/www/drupal.

The trivial fix is attached as a patch.

PS! This bug might be fix in 5.x-dev, but is still a problem in 4.x-dev.