Create a svndiff.sh file
mkdir ~/bin/
cp /dev/null ~/bin/svndiff.sh
chmod +x ~/bin/svndiff.sh
Copy the contents the following commands to your svndiff.sh
#!/bin/sh
DIFF="/usr/bin/opendiff"
LEFT=${6}
RIGHT=${7}
$DIFF "$LEFT" "$RIGHT"
Change your "diff-cmd"
nano -w ~/.subversion/config
Find the line with "# diff-cmd = diff_program (diff, gdiff, etc.)" and changed to:
diff-cmd = /Users/ctk/bin/svndiff.sh
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/opendiff.1.html