This might seem blatantly obvious in a minute but here is how to svn undo
Most people should be familiar with merging revisions from one code branch to another… you use svn merge
All svn merge does is make a unified diff from revision A to version B. So to merge revision 10 you would use: svn merge -r 9:10 svn://repository/path
The beauty of this is that you can tell it to give you the opposite diff by
switching the revision numbers (ie: svn merge -r 10:9 svn://repository/path) which gives you a diff that
undoes the changes in revision 10. (aka the diff necessary to go from revision 10 to revision 9)
(I learned this tip via svnbook - common use cases for merging)