git init server
cd server
touch a
git add .
git commit -m 0
git config --local receive.denyCurrentBranch updateInstead
cd ..
git clone server local
cd local
touch b
git add .
git commit -m 1
git push origin master:master
cd ../server
ls
# start the tunnel from the natted box you wish to pull from (local)
$ ssh -R 1234:localhost:22 user@remote
# on the other box (remote)
$ git remote add other-side ssh://user@localhost:1234/the/repo
$ git pull other-side