Helpfully, "svn help log" has a hint:
Logs follow copy history by default. Use --stop-on-copy to disable this behavior, which can be useful for determining branchpoints.
I've turned that idea into a handy wrapper script around "svn diff", which adds a revision specifier for "the revision just before this branch was created". I call it BRANCHPT or BRPT for short. Otherwise, the script just passes all arguments through unchanged to "svn diff".