What is the difference between civicrm-drupal/tree/8.x-master and civicrm-drupal-8?
The history of the branches started to diverge around Apr, 2018, and there's a comment on May 1 about creating a separate repo (https://github.com/civicrm/civicrm-drupal/pull/523#issuecomment-385612657):


  To facilitate distribution via composer, the Civi-D8 code is being migrated to a new repository with a slightly different branch naming convention (ie civicrm-drupal@8.x-master becomes civicrm-drupal-8@master).


So at the moment for drupal 8 the more current code to use for the drupal module part is at https://github.com/civicrm/civicrm-drupal-8.