svn diff -r HEAD ab
I think you need to specify a HEAD revision if something changes in the repository.
But now, when I re-read your question, it looks like you:
made changes
made a change
now I want to view these changes
In this case, you can get the current version number with
svn info ab
subtract 1 from it and use it in svn diff.
for example if your current revision is 100
svn diff -r 99: 100
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