Dont create redundant %config backups when on-disk content is same as new

Dont create redundant %config backups when on-disk content is same as new

- When on-disk %config file contents match the contents of the new
  package (such as config in old package was buggy, fixed in new
  and admin worked around in the meantime) there's no point creating
  a backup that's identical to what you already have. Note that
  we create the new config anyway instead of skipping as timestamp
  might have changed.
- Adjust test-cases to the new saner behavior.