[CMake] Best practices on how to reorganize repository

[Dieter Oberkofler]

Our SVN repository is growing very quickly and we are starting to have
problems with the backup of the fully repository and would therefore like to
reduce it in size by reorganizing it. What are the best practices (in order
of importance) to:
1) Physically remove branches and tags that are no longer needed?
2) Physically remove removed files (that should never have been in the
repository in the first place)?
3) Physically remove old history revisions?
Thank you,
Dieter