t5500: count objects through stderr, not trace

t5500: count objects through stderr, not trace

In two tests introduced by 4fa3f00abb ("fetch-pack: in protocol v2,
in_vain only after ACK", 2020-04-28) and 2f0a093dd6 ("fetch-pack: in
protocol v2, reset in_vain upon ACK", 2020-04-28), the count of objects
downloaded is checked by grepping for a specific message in the packet
trace. However, this is flaky as that specific message may be delivered
over 2 or more packet lines.

Instead, grep over stderr, just like the "fetch creating new shallow
root" test in the same file.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>