t5512: allow any protocol version for filtered symref test

t5512: allow any protocol version for filtered symref test

We have a test that checks that ls-remote, when asked only about HEAD,
will report the HEAD symref, and not others. This was marked to always
run with the v0 protocol by b2f73b70b2 (t5512: compensate for v0 only
sending HEAD symrefs, 2019-02-25).

But in v0 this test is doing nothing! For v0, upload-pack only reports
the HEAD symref anyway, so we'd never have any other symref to report.

For v2, it is useful; we learn about all symrefs (and the test repo has
multiple), so this demonstrates that we correctly avoid showing them.

We could perhaps mark this to test explicitly with v2, but since that is
the default these days, it's sufficient to just run ls-remote without
any protocol specification. It still passes if somebody does an explicit
GIT_TEST_PROTOCOL_VERSION=0; it's just testing less.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>