t5600: fix outdated comment about unborn HEAD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and this comment is out of date.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>

t/t5600-clone-fail-cleanup.sh

@@ -22,7 +22,7 @@ test_expect_success \
 # Need a repo to clone
 test_create_repo foo
 
-# clone doesn't like it if there is no HEAD. Is that a bug?
+# create some objects so that we can corrupt the repo later
 (cd foo && touch file && git add file && git commit -m 'add file' >/dev/null 2>&1)
 
 # source repository given to git clone should be relative to the