submodule: git_index_add_bypath does not move conflict entries to REUC for submodules

Because of the early return after git_submodule_add_to_index here, index_conflict_to_reuc is not called when the path is a submodule.

I can workaround this by calling git_index_conflict_remove after I call git_index_add_bypath.

I propose that git_submodule_add_to_index should call index_conflict_to_reuc (or something like it), as it is part of the API and should also handle conflicts correctly?