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
5726a6b (*.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
5726a6b for now, we can leave another weather balloon for some
other time.

1. http://lore.kernel.org/git/[email protected]

Signed-off-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: avar

cbtree.h

@@ -37,11 +37,12 @@ enum cb_next {
 	CB_BREAK = 1
 };
 
-#define CBTREE_INIT { .root = NULL }
+#define CBTREE_INIT { 0 }
 
 static inline void cb_init(struct cb_tree *t)
 {
-	t->root = NULL;
+	struct cb_tree blank = CBTREE_INIT;
+	memcpy(t, &blank, sizeof(*t));
 }
 
 struct cb_node *cb_lookup(struct cb_tree *, const uint8_t *k, size_t klen);