t1404: make D/F conflict tests compatible with reftable backend

t1404: make D/F conflict tests compatible with reftable backend

Some of the tests in t1404 exercise whether Git correctly aborts
transactions when there is a directory/file conflict with ref names.
While these tests are all marked to require the "files" backend, they do
in fact apply to the "reftable" backend as well.

This may not make much sense on the surface: D/F conflicts only exist
because the "files" backend uses the filesystem to store loose refs, and
thus the restriction theoretically shouldn't apply to the "reftable"
backend. But for now, the "reftable" backend artificially restricts the
creation of such conflicting refs so that it is a drop-in replacement
for the "files" backend. This also ensures that the "reftable" backend
can easily be used on the server side without causing issues for clients
which only know to use the "files" backend.

The only difference between the "files" and "reftable" backends is a
slightly different error message. Adapt the tests to accomodate for this
difference and remove the REFFILES prerequisite so that we start testing
with both backends.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>