Add a new per-repository variable, GIT_${REPO}_CHECKOUT_PATH,
which specifies the directory where the repository is
checked out in the sandbox.
Fix typos
Simplify do_checkout.sh to use GIT_${repo}_LOCAL_PATH variables
for specifying where to check out source trees.
Create a helper script which applies a
lightweight git tag to all checked out branches.
Rename create_tag.sh to apply_tag.sh
Add top level target:

make tag=<tagname>

will apply a tag "tagname"
to all checked out repos.