Flavour by sgilroy
In the terminal change to the repository's directory
cd /path/to/checkout
Add the appropriate project's upstream
git remote add upstream git://github.com/jenkinsci/jenkins.git
Optional refresh the upstream's data with
git fetch upstream
Later.
Add the remote origin via shell.
Click Version Control -> Git -> Pull Changes
For Remote, select upstream (default is origin)
Check the box next to master.
Click Pull.
Click Repository -> Repository Settings -> Add.
Name and fill the upstream url, e.g. upstream and git://github.com/jenkinsci/jenkins.git.
In the sidebar on the left side, choose the remote upstream fetch it data. Alternatively, choose Fetch (all) in the menu.
Right click on the master (or other branch) of the remote upstream and choose Pull upstream/master into master.