t1419: mark test suite as files-backend specific

With 59c35fac54 (refs/packed-backend.c: implement jump lists to avoid
excluded pattern(s), 2023-07-10) we have implemented logic to handle
excluded refs more efficiently in the "packed" ref backend. This logic
allows us to skip emitting refs completely which we know to not be of
any interest to the caller, which can avoid quite some allocations and
object lookups.

This was wired up via a new `exclude_patterns` parameter passed to the
backend's ref iterator. The backend only needs to handle them on a best
effort basis though, and in fact we only handle it for the "packed-refs"
file, but not for loose references. Consequently, all callers must still
filter emitted refs with those exclude patterns.

The result is that handling exclude patterns is completely optional in
the ref backend, and any future backends may or may not implement it.
Let's thus mark the test for t1419 to depend on the REFFILES prereq.

An alternative would be to introduce a new prereq that tells us whether
the backend under test supports exclude patterns or not. But this does
feel a bit overblown:

  - It would either map to the REFFILES prereq, in which case it feels
    overengineered because the prereq is only ever relevant to t1419.

  - Otherwise, it could auto-detect whether the backend supports exclude
    patterns. But this could lead to silent failures in case the support
    for this feature breaks at any point in time.

It should thus be good enough to just use the REFFILES prereq for now.
If future backends ever grow support for exclude patterns we can easily
add their respective prereq as another condition for this test suite to
execute.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Reviewed-by: Christian Couder <christian.couder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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