I know how to do it with SVN, but they've moved to Git.
The command for getting the newest version as described on their site is
git clone git://source.ffmpeg.org/ffmpeg.git ffmpeg
What would be the appropriate arguments for getting a specific revision?
In the cloned repository run:
git checkout <hash>
To return to the master branch run:
git checkout master
To find a particular hash you can use:
git log