t5601: mark protocol v2-only test

t5601: mark protocol v2-only test

A HTTP-clone test introduced in 4fe788b1b0 ("builtin/clone.c: add
--reject-shallow option", 2021-04-01) only works in protocol v2, but is
not marked as such.

The aforementioned patch implements --reject-shallow for a variety of
situations, but usage of a protocol that requires a remote helper is not
one of them. (Such an implementation would require extending the remote
helper protocol to support the passing of a "reject shallow" option, and
then teaching it to both protocol-speaking ends.)

For now, to make it pass when GIT_TEST_PROTOCOL_VERSION=0 is passed, add
"-c protocol.version=2". A more complete solution would be either to
augment the remote helper protocol to support this feature or to return
a fatal error when using --reject-shallow with a protocol that uses a
remote helper.

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