diff <commit> using 3-dot behavior

I want to view the complete diff of my branch (topic) relative to its
parent branch (master). This should include cached/staged files and
unstaged working tree changes.

If I do this:

$ git diff master

This will include changes on master *since* my last merge, which I do
not want (I don't want to see changes on master, only on topic). I
tried this:

$ git diff master --not master

This didn't give me any output. If I do this:

$ git diff master...topic

This shows me only committed changes on topic, but excludes staged &
unstaged changes.

How can I get the results I want?