Fixed #10847 -- Modified handling of extra() to use a masking strategy, rather than last-minute trimming. Thanks to Tai Lee for the report, and Alex Gaynor for his work on the patch.

This enables querysets with an extra clause to be used in an __in filter; as a side effect, it also means that as_sql() now returns the correct result for any query with an extra clause.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10648 bcc190cf-cafb-0310-a4f2-bffc1f526a37