Fully fix a bad rule object aliasing issue custom remarkup rules

Summary:
Fixes T10234. This is a more thorough fix.

Root issue is that some time around D13589, we started hitting an object cache for `loadCustomInlineRules()`, but didn't adjust the code to account for that.

So if a page created multiple similar engines, we'd return the same `$rule` object for multiple engines, call `setEngine()` on it with different engines, and then possibly try to render using an already-expired engine the second time through.

Instead, create a separate `$rule` object for each separate `$engine`.

Test Plan:
Repro is something like this:

  - Create a custominlinerule which uses an engine.
  - Purge the remarkup cache.
  - Load a page which uses the rule in two engines (e.g., in a revision description, and also in an inline comment).
  - Before change: second one could fatal. After change: clean load.

Reviewers: thoughtpolice, chad

Reviewed By: thoughtpolice, chad

Subscribers: thoughtpolice, eadler

Maniphest Tasks: T10234

Differential Revision: https://secure.phabricator.com/D15535

src/infrastructure/markup/PhabricatorMarkupEngine.php

@@ -517,7 +517,7 @@ final class PhabricatorMarkupEngine extends Phobject {
     $rules[] = new PhutilRemarkupHighlightRule();
 
     foreach (self::loadCustomInlineRules() as $rule) {
-      $rules[] = $rule;
+      $rules[] = clone $rule;
     }
 
     $blocks = array();