mercurial-repository: implement BackupAdditional and RestoreAdditional

Also added the tests referenced/copied from py-heptapod side testing
of `hpd-backup-additional` and `hpd-restore-additional` commands.
4 jobs for !69 with topic/default/backup-additional in 2 minutes and 2 seconds (queued for 3 seconds)
latest detached
Status Job ID Name Coverage
  Main
passed tests-current #240472

00:01:30

100.0%
 
  Compat
passed check-sdist #240473

00:00:54

passed tests-hg-default #240475

00:00:31

100.0%
passed tests-hg-stable #240474

00:00:30

100.0%