One of the problems I’ve been working with for some time with Git is using a single clone of a submodule with multiple containing repositories. I tried using symbolic links, but Git refuses to interpret the symbolic links as real submodules. Then I found an interesting workaround via the Git configuration:

1. Set up the repositories individually, as you would normally do.
2. Choose a location to place the submodule. Place exactly one checkout of the submodule there.
3. Delete the submodule directory from all other locations. In their place, add an empty directory (for compatibility reasons).
4. Run git config --local submodule.SubmoduleName.path /path/to/submodule in each repository you want to share the submodule between. The path should either be relative to the repository root or absolute.

Now you have a single checkout of the submodule shared among all repositories. I made a sample repository using SpongeForge and SpongeVanilla; you can view it here. See the README file for usage instructions; the init script contains all the logic.