Bug 21579: Make showdiffmarc.pl work for authorities and biblios

Bug 21579: Make showdiffmarc.pl work for authorities and biblios

To test:
 1 - Have or make some authority records
 2 - Have or make an authority matching rule
 3 - Export your authrotities
 4 - Import them using the matching rule
 5 - On the 'Manage staged records' page view some diffs
 6 - The import record is correct, but the existing records pull form
 bibs
 7 - Apply patch
 8 - Repeat
 9 - Confirm you see the expected authority matches

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marjorie Vila <marjorie.barry-vila@collecto.ca>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>