Bug 25313: Add skip_merge to ModAuthority

Bug 25313: Add skip_merge to ModAuthority

Sometimes you know that the merge is useless, since the reporting
tag did not change.
Might be handy in batch processing of authority records.

Test plan:
Pick an authority with a few linked biblio recs (so that it will
immediately merge; lower than MergeLimit).
Open authority record and save it.
Verify that a linked biblio record was merged. Check time in 005.
You proved that this patch does not break the regular process.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Phil Ringnalda <phil@chetcolibrary.org>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>