cbtree.h: define cb_init() in terms of CBTREE_INIT

Use the same pattern for cb_init() as the one established in the
recent refactoring of other such patterns in
5726a6b4012 (*.c *_init(): define in terms of corresponding *_INIT
macro, 2021-07-01).

It has been pointed out[1] that we could perhaps use this C99
replacement of using a compound literal for all of these:

    *t = (struct cb_tree){ 0 };

But let's just stick to the existing pattern established in
5726a6b4012 for now, we can leave another weather balloon for some
other time.

1. http://lore.kernel.org/git/ef724a3a-a4b8-65d3-c928-13a7d78f189a@gmail.com

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

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