LP#1406786: fix remapping copies during parts merge

LP#1406786: fix remapping copies during parts merge

This patch fixes an issue with how copies are mapped
on the target part when using "Merge Selected" in the
Monograph Parts interface.

To test:

[1] Create some monograph parts on a record and assign
    some copies to use them.
[2] Use the merge parts function to collapse all of the
    parts created in step 1 onto a single part.
[3] Verify that the copies whose original parts were removed
    are now mapped onto the part that was the target of the
    merge.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>