fetch test: mark test of "skipping" haves as v0-only

fetch test: mark test of "skipping" haves as v0-only

Since 633a53179e (fetch test: avoid use of "VAR= cmd" with a shell
function, 2019-12-26), t5552.5 (do not send "have" with ancestors of
commits that server ACKed) fails when run with
GIT_TEST_PROTOCOL_VERSION=2.

The cause:

The progression of "have"s sent in negotiation depends on whether we
are using a stateless RPC based transport or a stateful bidirectional
one (see for example 44d8dc54e7, "Fix potential local deadlock during
fetch-pack", 2011-03-29).  In protocol v2, all transports are
stateless transports, while in protocol v0, transports such as local
access and ssh are stateful.

In stateful transports, the number of "have"s to send multiplies by
two each round until we reach PIPESAFE_FLUSH (that is, 32), and then
it increases by PIPESAFE_FLUSH each round.  In stateless transport,
the count multiplies by two each round until we reach LARGE_FLUSH
(which is 16384) and then multiplies by 1.1 each round after that.

Moreover, in stateful transports, as fetch-pack.c explains:

	We keep one window "ahead" of the other side, and will wait
	for an ACK only on the next one.

This affects t5552.5 because it looks for "have"s from the negotiator
that appear in that second window.  With protocol version 2, the
second window never arrives, and the test fails.

Until 633a53179e (2019-12-26), a previous test in the same file
contained

	GIT_TEST_PROTOCOL_VERSION= trace_fetch client origin to_fetch

In many common shells (e.g. bash when run as "sh"), the setting of
GIT_TEST_PROTOCOL_VERSION to the empty string lasts beyond the
intended duration of the trace_fetch invocation.  This causes it to
override the GIT_TEST_PROTOCOL_VERSION setting that was passed in to
the test during the remainder of the test script, so t5552.5 never got
run using protocol v2 on those shells, regardless of the
GIT_TEST_PROTOCOL_VERSION setting from the environment.  633a53179e
fixed that, revealing the failing test.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>