New option --remove-unversioned-files to cleanup

Summary:
I would like to have a new option --remove-unversioned-files to svn cleanup. When calling 

  svn cleanup --remove-unversioned-files PATH

using this option it would -- in addition to cleanup already performed today -- traverse the working 
copy at PATH and remove all unversioned files and directories found.

Usecase:
My primary usecase where I would like to be able to use this new option is a period merge of trunk into 
a branch. Best practices for this use case require having a clean, up-to-date working copy of the 
branch. Often, after cleaning up and updating, my working copy contains artifacts created from 
automatic tests, which I neither want to keep nor commit into the repository. In this situation it would 
be very handy to have this option.

Other solutions:
Many users I have talked to have written custom scripts performing this task such as

  svn status | grep '^?' | awk '{print $2}' | xargs rm -rf

I argue that svn support of --remove-unversioned-files would be a safer alternative. Scripts like the 
one above are error prone (this one contains a bug when the working copy contains paths with spaces).

Other users checkout separate working copies which are used exclusively for merging.

References:
http://subversion.tigris.org/ds/viewMessage.do?dsForumId=1065&dsMessageId=2429229 

It would be awesome if this enhancement made it into a future version of subversion.

Original issue reported by tobiashahn