t5605: fix test for cloning from a different user

t5605: fix test for cloning from a different user

This test currently passes, but for the wrong reason.  The
repo_is_hardlinked function expects a .git directory or a bare
repository and currently fails because it cannot find the objects
directory.

One solution is to use the --bare argument, but then --show-toplevel
won't work.  We could change that, but there's no need to, so just add
the missing .git directory.

In addition, use the built-in negation functionality of test_grep to
avoid mishandling real errors (such as a missing file) and, as a final
fix, remove the extra newline.

Reported-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>