t5552: compensate for v2 filtering ref adv.

Protocol v2 filters the ref advertisement, but protocol v0 does not. A
test in t5552 uses the ref advertisement, so fix it to use protocol v0.

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

t/t5552-skipping-fetch-negotiator.sh

@@ -127,7 +127,10 @@ test_expect_success 'use ref advertisement to filter out commits' '
 	# not need to send any ancestors of "c3", but we still need to send "c3"
 	# itself.
 	test_config -C client fetch.negotiationalgorithm skipping &&
-	trace_fetch client origin to_fetch &&
+
+	# The ref advertisement itself is filtered when protocol v2 is used, so
+	# use v0.
+	GIT_TEST_PROTOCOL_VERSION= trace_fetch client origin to_fetch &&
 	have_sent c5 c4^ c2side &&
 	have_not_sent c4 c4^^ c4^^^
 '