The Drubuntu Drupal development environment for Ubuntu already installed Drush in my workspace at ~/workspace/drush, so when cloning the Drush project to work on it i jokingly called it drush8 since the goal of my patch is Drupal 8 compatibility for the site-install command.
git clone --branch master http://git.drupal.org/project/drush.git drush8
git checkout -b 1111086-drush-site-install-drupal-8-compatibility
# Make changes, for instance gvim commands/core/core.drush.inc and others.
git add .
git commit -m "Drupal 8 compatibility (for key core contributor commands, starting with site-install)."
# If it took you a while to make those changes, and even if not,
# it is a good habit to check that your changes still apply:
git fetch origin
git rebase origin/master
git format-patch origin/master --stdout > 1111086-drush-site-install-drupal-8-compatibility-1.patch
Now after all that automated command line goodness, you still have to go to your browser and attach your patch to the issue!