copyright alias for individual revision

[Peter Johansson]

As now one can set copyright alias in config file, and the setting is global in the sense it applies to all revisions. There is a use case when a bit more flexibility is desired. Say that someone submit a patch via email (or ​Trac), then perhaps oen would like to keep the patch writer as copyright holder. If the maintainer (who submits the patch to svn repo) make significant changes to the patch, he possibly should also hold copyright for the patch, so possibly we should allow multiple copyright holders for one revision.