refs: use designated initializers for "struct ref_storage_be"

refs: use designated initializers for "struct ref_storage_be"

Change the definition of the three refs backends we currently carry to
use designated initializers.

The "= NULL" assignments being retained here are redundant, and could
be removed, but let's keep them for clarity. All of these backends
define almost all fields, so we're not saving much in terms of line
count by omitting these, but e.g. for "refs_be_debug" it's immediately
apparent that we're omitting "init" when comparing its assignment to
the others.

This is a follow-up to similar work merged in bd4232fac33 (Merge
branch 'ab/struct-init', 2021-07-16), a4b9fb6a5cf (Merge branch
'ab/designated-initializers-more', 2021-10-18) and a30321b9eae (Merge
branch 'ab/designated-initializers' into
ab/designated-initializers-more, 2021-09-27).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>