t5551: mark half-auth no-op fetch test as v0-only

t5551: mark half-auth no-op fetch test as v0-only

When using protocol v0, upload-pack over HTTP permits a "half-auth"
configuration in which, at the web server layer, the info/refs path is
not protected by authentication but the git-upload-pack path is, so that
a user can perform fetches that do not download any objects without
authentication, but still needs authentication to download objects.

But protocol v2 does not support this, because both ref and pack are
obtained from the git-upload-pack path.

Mark the test verifying this behavior as protocol v0-only, with a
description of what needs to be done to make v2 support this.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>