how to reduce disk usage for large .git dirs?

How can I reduce the disk usage for multiple copies of the same repo?

Up to now I just made copies like this, but since .git alone is already
2GB it becomes expensive:

 # git clone git://host/repo.git repo-master
 # cp -a repo-master repo-branchA
 # cd repo-branchA
 # git checkout -b branchA origin/branchA
 # cd -
 # cp -a repo-master repo-branchB
 # cd repo-branchB
 # git checkout -b branchB origin/branchB
 # cd -
 # cp -a repo-master repo-branchB-feature
 # cd repo-branchB-feature
 # git checkout -b branchB-feature origin/branchB
 # cd -


Since each .git is almost identical I wonder if there is a reliable way
to "share" it. The "git clone" man page mentions --shared as a dangerous
way to do things. It does not give an advice how to manage such cloned
trees.

So how can I reduce the disk usage needed for the four .git dirs above?
I looked around in the docs that came with my git-2.1.3 package, but
found nothing that answers my question. Maybe we can workout something
and add it to one of the existing docs.

Thanks!

Olaf