bug #19986 [Form][EventDispatcher] Fix VarDumper usage related to perf regression (nicolas-grekas)

This PR was merged into the 3.2-dev branch.

Discussion
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[Form][EventDispatcher] Fix VarDumper usage related to perf regression

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #19978
| License       | MIT
| Doc PR        | -

Commits
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294868e [Form][EventDispatcher] Fix VarDumper usage related to perf regression

Author: fabpot

DataCollector/DataCollector.php

@@ -67,6 +67,7 @@ protected function cloneVar($var)
         if (null === $this->cloner) {
             if (class_exists(ClassStub::class)) {
                 $this->cloner = new VarCloner();
+                $this->cloner->setMaxItems(250);
                 $this->cloner->addCasters(array(
                     Stub::class => function (Stub $v, array $a, Stub $s, $isNested) {
                         return $isNested ? $a : StubCaster::castStub($v, $a, $s, true);