Bug 28217: Prevent several non-repeatable attributes to be merged

Bug 28217: Prevent several non-repeatable attributes to be merged

When using the patron merge feature it's possible to generate a patron
with several non-repeatable attributes.

This patch prevents that.

Test plan:
Create 2 patron attribute types, one repeatable and one non-repeatable
Create 2 patrons and add them repeatable attributes
Add a non-repeatable attribute to one of them
Merge the 2 patrons
=> It succeeds, the resulting patron has all the repeatable attribute
and the non-repeatable one.

Do it again but this time add non-repeatable to both patrons
Merge them
=> It fails, you should see an error on the UI
  Merge failed! The following error was reported: Tried to add more than one non-repeatable attributes. type=TYPE value=VALUE.

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>