I do this as a two-step process. First, I find the version that was the origin of the branch. From within the checkout of the branch:
svn log --stop-on-copy |tail -4
--stop-on-copy tells SVN to only operate on entries after the branch. tail gets you the last log entry, which is the one that contains the branch information. The number that begins with an 'r' is the revision at which you branched. Then, use svn diff to find changes since that version:
svn diff -r <revision at which you branched>:head --summarize
the --summarize option shows a file list only, without the actual diff contents, similar to the 'svn status' output. If you want to see the actual diff, just remove the --summarize option.