Keep parent pointers when update() merges past its descent bound

The iterative path of update() only set the parent pointer of the member
it inserted, like the recursive one did before. It now uses set_parent()
too, so ordered_json members that move when a nested object grows keep
their parents, and the set_parents() calls that patched this up after
each nested merge are gone.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-diagnostics.cpp

@@ -422,6 +422,28 @@ TEST_CASE("Regression tests for extended diagnostics")
             check_parents(j);
         }
 
+        // update(j, true) around its descent bound, where the nested vectors
+        // grow while the objects are merged without recursing
+        for (const std::size_t depth :
+                {
+                    nlohmann::detail::recursion_depth_limit() - 1, nlohmann::detail::recursion_depth_limit(), nlohmann::detail::recursion_depth_limit() + 2
+                })
+        {
+            ordered_json j = {{"z", {{"x", 1}}}};
+            ordered_json patch = {{"a", 1}, {"b", 2}, {"c", {{"d", 3}}}};
+            for (std::size_t i = 0; i < depth; ++i)
+            {
+                j = ordered_json{{"k", 0}, {"n", std::move(j)}};
+                patch = ordered_json{{"n", std::move(patch)}, {"l", 1}, {"m", 2}};
+            }
+            j.update(patch, true);
+
+            // must not trigger assert_invariant() on any level in a
+            // debug/assert-enabled build
+            ordered_json const copy = j; // NOLINT(performance-unnecessary-copy-initialization)
+            CHECK(copy == j);
+        }
+
         // merge_patch() inserts "c" and removes "d" at /a/c, then inserts "e"
         // at /a, which copies /a/c
         {