Makefile

%.git.push: %.git
  cd $*.git && git fetch origin && git push github  2>&1 >> ~/github.log

%.git:
	[ -e $*/config ] || git clone --mirror git@private-git-host:$* \
	&& cd $*.git && git remote add --mirror=push github [email protected]:PrivateOrganization/$*

#put this Makefile in github_mirror@yourserver:~/repos/
#Then copy the following 2 lines into private-git-hosts's post-receive hook for the repo:
#GITDIRNAME=pwd | perl -w -ne 'my ($dir) = reverse split(/\//); print $dir'
#ssh github_mirror@yourserver "cd repos && make $GITDIRNAME.push"