update-index: allow overwriting existing submodule index entries

In commit e01105 Linus introduced gitlinks to update-index. He explains that he thinks it is not the right thing to replace a gitlink with something else. That commit is from the very first beginnings of submodule support. Since then we have gotten a lot closer to being able to remove a submodule without losing its history. This check prevents such a use case, so I think this assumption has changed. Additionally in the git add codepath we do not have such a check, so for consistency reasons I think removing this check is the correct thing to do. Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>