In 47506af, serializing references content to be added to data-mw was
delayed until we were inserting the reference into the dom, to give us a
chance to mark up errors about not finding named ref content. After
which, the content is cleaned out of references to make room for list
items that belong there.
In 6bd0594, we noted that we can't store linkback from embeded content
since the content is released after serializing and won't be available
when it comes time to mark it up with errors.
Similarly, the linkbacks added to other groups from the references
content won't be available after inserting that reference and so we
shouldn't be holding on to them either. This means that we won't be
marking them up for the named error above but with the benefit that we
won't crash when trying to access these linkbacks from another group.
A test case is added which crashes trying to access a linkback from a
group that has already been inserted.
Note that the extension to the legacy parser considers referencing
another group from group content to be
"cite_error_references_group_mismatch". If we choose to also do that,
we may be able to reverse this change.
Change-Id: Idf0e49fa07dc3614068793c72a30ce3de1e2392c
b8d4bb1f37