t1400: split up generic reflog tests from the reffile-specific ones

t1400: split up generic reflog tests from the reffile-specific ones

We have a bunch of tests in t1400 that check whether we correctly read
reflog entries. These tests create the reflog by manually writing to the
respective loose file, which makes them specific to the files backend.
But while some of them do indeed exercise very specific edge cases in
the reffiles backend, most of the tests exercise generic functionality
that should be common to all backends.

Unfortunately, we can't easily adapt all of the tests to work with all
backends. Instead, split out the reffile-specific tests from the ones
that should work with all backends and refactor the generic ones to not
write to the on-disk files directly anymore.

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