I notice that in contrib/tools/post-review:SVNClient.diff it calls
"svn diff %s".  This means that it invokes svn in a way that loads
~/.subversion/config.  This means that the diff-cmd I have set in my
~/.subversion/config gets invoked.  I use tkdiff, and so instead of
finding the patch file on stdout, there is nothing.

It seems like there are a few possible solutions:
1.  set --diff-cmd to something reasonable.  This is probably hard
across OS'es (I'm guessing that diff(1) isn't on most people's path on
windows).
2.  I'd love to run svn --diff-cmd=built-in, but it seems like svn
doesn't have that.  Perhaps there's a way to set an environment
variable to point the subversion from config to ~/.subversion/config
to /this/is/not/a/file?
3.  Some other way?

uploaded this patch with my modified post-review