Don't bother calculating digests of %ghost %config files

Don't bother calculating digests of %ghost %config files

- Calculating digest of %ghost config is useless as there's nothing
  to compare the result to. Also we never take backups of %ghost
  configuration anyway, so this is a total waste of time. One common
  case of %ghost %config is the rpmdb (environment and all) which
  can be rather large and calculating digests can take several seconds,
  only for the results to be thrown away unused.
- There are some cases where it might be reasonable to back up %ghost
  %config (eg if it gets replaced by non-config), but
  rpmfiConfigConflictIndex() doesn't have sufficient context to figure
  that out. For now, preserve the traditional simple rule: no backups
  for ghosts, ever.