Hmm - the "official" git PPA isn't updated yet: https://launchpad.net/~git-core/+archive/ubuntu/ppa. Wonder if this due to dependencies or oversight?
echo "deb http://ftp.debian.org/debian stretch-backports main" | tee /etc/apt/sources.list.d/stretch-backports.list
apt-get update
apt-get -t stretch-backports install git