GitPython has a problem while listing submodules.

As '.gitmodules' contains all the modules have ever been registered, it becomes problematic when some of them are removed. 'repo.submodules' will fail to work in this situation, though, 'repo.iter_submodules()' still works. But when 'repo' itself is a submodule, 'repo.iter_submodules()' does not yield any result.