t1404: mark directory/file conflict tests with REFFILES

t1404: mark directory/file conflict tests with REFFILES

The files backend uses file system locking on individual refs, which means a
directory/file conflict can prevent locks being taken. For example, in a repo
with just the ref "foo", an update

    (DELETE "foo") + (ADD "foo/bar")

cannot be executed in the files backend, as one cannot take a lock on foo/bar.

The current reftable proof-of-concept integration supports these tranactions, as
the result is a repo with just "foo/bar", which has no directory/file conflict.

Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>