Support of redundant non-generated content div

We do not support redundant non-generated content div when handling Macro content. So for example in a code such as:

<!--startmacro:box|-|Old content-->
<div class="box">
<div data-xwiki-non-generated-content="java.util.List&lt;org.xwiki.rendering.block.Block&gt;" class="xwiki-metadata-container">
<div data-xwiki-non-generated-content="java.util.List&lt;org.xwiki.rendering.block.Block&gt;" class="xwiki-metadata-container">
<p>Some content</p>
</div>
</div>
</div>
<!--stopmacro-->

all the content will be ignored right now. I propose to fix that to ignore those redundant unchanged-content div in such case.
This can be especially helpful for XWIKI-16389 when adding a wikimacro inline editable content inside another inline editable macro, such as a box.