Never mind. I'll postprocess it myself. How silly of me. What could be simpler?
svn diff -r BASE:HEAD RELEASE-NOTES|
perl -nlwe 'next if 1../^\@\@/;print if /^[-+]/'|
perl -we 'local $/; while(<>){s/\n[+\- ] / /g;print}'|
perl -nwle 's/\s+$//;tr/ //s;next unless length;/(.)(.*)/;$h{$2}+=(eval $1 . 1);
END{for (sort keys %h){next unless $h{$_}; print $h{$_} > 0 ?
"+":"-", $_}}'