t1404: mark tests that muck with .git directly as REFFILES.

t1404: mark tests that muck with .git directly as REFFILES.

The packed/loose ref storage is an overlay combination of packed-refs (refs and
tags in a single file) and one-file-per-ref. This creates all kinds of edge
cases related to directory/file conflicts, (non-)empty directories, and the
locking scheme, none of which applies to reftable.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>