The main git repo includes contrib/workdir/, which is a command that allows sharing objects in the .git/ folder between repos. This is useful on jenkins and other platforms where you might have the same repo checked out in several different directories.
Newer versions of git have added support for this functionality in a different way, via GIT_ALTERNATE_OBJECT_DIRECTORIES. But it doesn't work very well with submodules. Neither does the original git-workdir. But this version does!