1) Deploy Key (https://help.github.com/articles/managing-deploy-keys#deploy-keys)

Create a SSH key for the user that will sync https://help.github.com/articles/generating-ssh-keys

ssh-keygen -t rsa -C "syncupp@ultimatepp.org"
- in upp_id_rsa
- no pass phrase

$ ssh-add ~/.ssh/upp_id_rsa

Go to https://github.com/ultimatepp/upp/settings/keys & paste the pub key

2) git-svn-mirror (https://github.com/alloy/git-svn-mirror)

$ gem install git-svn-mirror

instructions (https://github.com/alloy/git-svn-mirror)
$ mkdir -p /path/to/workbench
$ git-svn-mirror init --from=http://upp-mirror.googlecode.com/svn --to=git@github.com:ultimatepp/upp.git --workbench=/path/to/workbench

It will take quite some time to create the workbench... probably a couple of hours depending on your connection.

3) Update the CRON

Create upp-update in /etc/cron.hourly/ with the following content:
#!/bin/sh
/usr/local/bin/git-svn-mirror update /path/to/workbench >/dev/null 2>&1