diff between working copy dirs?

This seems like it should be a FAQ, but while I saw a few related posts in
the archives, there didn't seem to be anything definitive. Apologies if I
just missed it.
Anyway, I'm trying to do e.g.
svn diff branches/my_branch trunk

But it always reports no differences (and no error message, either).
 Whereas if I try

svn diff --old=branches/my_branch --new=trunk

I get an error message about a usage that is "not yet supported".

It seems that my options are limited to either using full repository URLs
for both targets, or using a local shell diff (with appropriate flags to
exclude the contents of the .svn directories). Is that correct?

If so, what's the rationale for the limitation - potential confusion between
the contents of the working directories and the contents of the
corresponding repository URLs?

It just seems like the sort of thing that "should work", but of course
that's easy to say from the perspective of someone who hasn't tried to
implement it.

Thanks.

-- 
Mark J. Reed <markjreed_at_gmail.com>
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