Acq general search: improve searching for negative comparisons

This aims to address Launchpad bug #1031535.  I think the bug only
really shows up when searches involve invoices either as the core type
or with filter fields.  If a search doesn't involve invoices, the
problematic joins aren't present.

Could use more testing though.  Seems to make the problem go away, and
other basic searches seem to work, but I certainly haven't tested
everything.

Signed-off-by: Lebbeous Fogle-Weekley <lebbeous@esilibrary.com>
Signed-off-by: Dan Wells <dbw2@calvin.edu>