git clone: don't clone into non-empty directory

git clone: don't clone into non-empty directory

When using git clone with --separate-git-dir realgitdir and
realgitdir already exists, it's content is destroyed.

So, make sure we don't clone into an existing non-empty directory.

When d45420c1 (clone: do not clean up directories we didn't create,
2018-01-02) tightened the clean-up procedure after a failed cloning
into an empty directory, it assumed that the existing directory
given is an empty one so it is OK to keep that directory, while
running the clean-up procedure that is designed to remove everything
in it (since there won't be any, anyway).  Check and make sure that
the $GIT_DIR is empty even cloning into an existing repository.

Signed-off-by: Ben Wijen <ben@wijen.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>