t5544: clarify 'hook works with partial clone' test

t5544: clarify 'hook works with partial clone' test

Apply a few leftover improvements from the review of ad5df6b782
(upload-pack.c: fix filter spec quoting bug).

1. Instead of enumerating objects reachable from HEAD, enumerate all
reachable objects, because HEAD has not special significance in this
test.

2. Instead of relying on the knowledge that "? in rev-list output
means partial clone", explicitly verify that there are no blobs with
cat-file.

Signed-off-by: Jacob Vosmaer <jacob@gitlab.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>