t-reftable-tree: improve the test for infix_walk()

In the current testing setup for infix_walk(), the following
properties of an infix traversal of a tree remain untested:
- every node of the tree must be visited
- every node must be visited exactly once
In fact, only the property 'traversal in increasing order' is tested.
Modify test_infix_walk() to check for all the properties above.

This can be achieved by storing the nodes' keys linearly, in a nullified
buffer, as we visit them and then checking the input keys against this
buffer in increasing order. By checking that the element just after
the last input key is 'NULL' in the output buffer, we ensure that
every node is traversed exactly once.

Mentored-by: Patrick Steinhardt <ps@pks.im>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Chandra Pratap <chandrapratap3519@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/t-reftable-tree.c

@@ -15,15 +15,14 @@ static int t_compare(const void *a, const void *b)
 }
 
 struct curry {
-	void *last;
+	void **arr;
+	size_t len;
 };
 
-static void check_increasing(void *arg, void *key)
+static void store(void *arg, void *key)
 {
 	struct curry *c = arg;
-	if (c->last)
-		check_int(t_compare(c->last, key), <, 0);
-	c->last = key;
+	c->arr[c->len++] = key;
 }
 
 static void t_tree_search(void)
@@ -55,15 +54,24 @@ static void t_infix_walk(void)
 {
 	struct tree_node *root = NULL;
 	void *values[11] = { 0 };
-	struct curry c = { 0 };
+	void *out[11] = { 0 };
+	struct curry c = {
+		.arr = (void **) &out,
+	};
 	size_t i = 1;
+	size_t count = 0;
 
 	do {
 		tree_search(&values[i], &root, t_compare, 1);
 		i = (i * 7) % 11;
+		count++;
 	} while (i != 1);
 
-	infix_walk(root, &check_increasing, &c);
+	infix_walk(root, &store, &c);
+	for (i = 1; i < ARRAY_SIZE(values); i++)
+		check_pointer_eq(&values[i], out[i - 1]);
+	check(!out[i - 1]);
+	check_int(c.len, ==, count);
 	tree_free(root);
 }