Process only latest differential backup during restore

If a backup set includes only full + differential backups, the restore process applies changes for
all existent differential backups, instead of only taking the full + latest diff.

In case a backup set includes mixed full + inc + diff backups, that is ok, but if only full + diff backups are
included, its sufficient to take the latest differential only.

This could speed up restore times.

* Check if backup set includes only differential backups
* Use Full + latest differential backup during restore

Current Workaround: use the --sequence option during restore and pass full + latest diff as parameter.