Get the locations of submodule information.

This is a bit like a very lightweight version of git_submodule_status. It just returns a made of the first four submodule status values (i.e. the ones like GIT_SUBMODULE_STATUS_IN_HEAD, etc) that tell you where the submodule data comes from (i.e. the HEAD commit, gitmodules file, etc.). This can be useful if you want to know if the submodule is present in the working directory at this point in time, etc.