Copying a document updates the parent field of the source's (parent-child) children

How to reproduce:

1. Create a document A
2. From A, create a document B
3. View document B with ?viewer=xml in the URL, view page source and search for <parent>A.WebHome</parent>
4. Copy A to C
5. View again document B with ?viewer=xml in the URL, view page source and search for <parent>xwiki:C.WebHome</parent>. The parent should not have changed.