t5600: fix outdated comment about unborn HEAD

t5600: fix outdated comment about unborn HEAD

Back when this test was written, git-clone could not handle
a repository without any commits. These days it works fine,
and this comment is out of date.

At first glance it seems like we could just drop this code
entirely now, but it's necessary for the final test, which
was added later. That test corrupts the repository by
temporarily removing its objects, which means we need to
have some objects to move.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>