#701 Unmerge variant bug

[Ahasuerus]

The unmerge process works incorrectly when the following conditions are satisfied:

1. A publication has two titles; one is the parent of the other. It typically happens with COVERART/INTERIORART pairs, but it also happens with other title types.
2. An editor unmerges the parent title from the other titles it is merged with

The result will be TWO new title records rather than the expected one. In addition, both new titles will be in the publication, i.e. the publication goes from having a single parent title record to two duplicate title records.

[Ahasuerus]

Fixed in common/SQLparsing.py , installed in SVN 115 on 2018-05-11. Closing.