I don’t know if you want to introduce complexity from git into your codebase, I personally wouldn’t consider this as a viable option.
Depending on how your projects are organized, I would recommend to have the shared code separated in a library, but in the same repo (a monorepo that hosts both the projects and the library). In this way you can use the option of local dependency and not worry about maintaining separate variants for the library codebase and version them:
defp deps do
[
{:local_dependency, path: "path/to/local_dependency"}
]
end
