What svn command would list all the files modified on a branch?

You can also get a quick list of changed files if thats all you're looking for using the status command with the -u option

svn status -u

This will show you what revision the file is in the current code base versus the latest revision in the repository. I only use diff when I actually want to see differences in the files themselves.

There is a good tutorial on svn command here that explains a lot of these common scenarios: SVN Command Reference

This will do it I think:

svn diff -r 22334:HEAD --summarize <url of the branch>