I followed the git instructions at: https://www.drupal.org/project/drupal/git-instructions to get Drupal 8 :
git clone --branch 8.0.x http://git.drupal.org/project/drupal.git .
I then checked out the tag for 8.0.0 beta 6:
git checkout tags/8.0.0-beta6
And this command confirms I'm on that tag:
$ git name-rev --tags --name-only $(git rev-parse HEAD)
8.0.0-beta6
I then prepared for the install: set up a blank database and user and then used this drush command to do the install:
drush site-install standard --account-name=admin --account-pass=admin --db-url=mysql://d8:d8@localhost/d8
When I go into the admin - admin/reports/status - I'm told that I'm using 8.0.0-dev and that Unknown release date (version 8.0.0-beta6 available) is available.
But why? when I was already on 8.0.0-beta6 branch before I ran the install.