How to install drush 8 properly on Debian 8?
(Updated to fix links and clarify Drush 8 vs 9.)

Debian still (as of February 8th 2018) comes with Drush 5.10 which is completely unacceptable.

There is no reason to use Git to download Drush. Either 


If you want the old Drush 8 (that works for Drupal 8.3 and older, including Drupal 7) download a single PHAR file using the the straight forward instructions on http://docs.drush.org/en/8.x/install/ or
If you want the current Drush 9, install a site-local Drush (with Composer) and Drush Launcher (as a PHAR file) as described on http://docs.drush.org/en/master/install/