Bug 18071: (QA follow-up) Add commit parameter, add warning

Bug 18071: (QA follow-up) Add commit parameter, add warning

Addressing points mentioned in comment12:
[1] Commit parameter.
[2] Warning if authid does not exist for -merge.

Test plan:
[1] Run update_authorities.pl -authid X -merge -ref Y -c
    where X does not exist in your db and Y does.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>