t1400: exercise reflog with gaps with reftable backend

t1400: exercise reflog with gaps with reftable backend

In t1400, we have a test that exercises whether we print a warning
message as expected when the reflog contains entries which have a gap
between the old entry's new object ID and the new entry's old object ID.
While the logic should apply to all ref backends, the test setup writes
into `.git/logs` directly and is thus "files"-backend specific.

Refactor the test to instead use `git reflog delete` to create the gap
and drop the REFFILES prerequisite.

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