Code review comment for lp:~spiv/bzr/insert-stream-check-chks-part-2

This completes the fix for bug 406687.

We could go much further I think in terms of reusing code. I've taken a nibble at that in this branch but there's room to do much, much better. We now have three code paths in groupcompress_repo.py that are doing the essentially the same logic of finding text references from a set of revisions:

 - GroupCHKStreamSource,
 - fileids_altered_by_revision_ids, and
 - _check_new_inventories (the new checks in this patch).

However I think the incremental improvement in this patch is landable without tackling all the duplication at once. (But I'm happy to do any more low-hanging fruit that a reviewer considers appropriate.)