Bug 15682 - Merging records from cataloguing search only allows to merge 2 records

Bug 15682 - Merging records from cataloguing search only allows to merge 2 records

To test:
1 - Perform a cataloging search
2 - Attempt to merge 0 results - should fail
3 - Attempt to merge 1 resutls - should fail
4 - Attempt to merge 2 results - should succeed
5 - Attempt to merge 3 results - should succeed
6 - Test any other amount of records and if more than 1 it should
succeed

**Note: On shelves.pl you can merge a single record.  I think that is
incorrect so made this only work for 2. Will add a followup to fix
shelves.pl

Signed-off-by: Chris Cormack <chrisc@catalyst.net.z>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher <bredan@bywatersolutions.com>
(cherry picked from commit 8f1e5ad95fd78cbf09028e3d2dfe0b2b77d4dd21)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>