Bug 10648 - In records merge greatest field can not be added

Bug 10648 - In records merge greatest field can not be added

When merging 2 records (/cgi-bin/koha/cataloguing/merge.pl), the destination record is build using the fields and subfields checked in source records.
When a field is checked, the javascript code searches in destination record a field with a greater tag number to insert new field before.
When the new field tag number is greater than all existing field tag numbers, the field is not added.

This patch corrects this by adding at end if no greater field tag number exists. Also adds a sort of fields by tag number because all mergo code is based on this.

Test plan :
- Add to a framework a repeatable field with the greater non existing tag number. For example 998.
- Edit 2 records with this framework and add them a value in this tag.
- Put those records is a list
- Go to this list and check the two records
- Click on "Merge selected"
- Click on next
- Go to second source record
- Click on the greatest tag number. for example 998.
=> The field is added at the end of destination record

Signed-off-by: Nick Clemens <nick@quecheelibrary.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Works as described, no regressions.