Following up on this, the tag has now been updated (so the normal instructions should work fine):
git clone https://github.com/your-github-userid/ardupilot
cd ardupilot
# fetch the latest tags
git fetch --tags
# checkout the latest stable tag, and branch off it in your own 'new-branch'
git checkout ArduSub-stable -b new-branch
# update submodules
git submodule update --init --recursive
# install prerequisites
Tools/environment_install/install-prereqs-ubuntu.sh -y
# reload the path
. ~/.profile
# configure waf for your board (e.g. Pixhawk1)
./waf configure --board Pixhawk1