"Which we discussed" vs. "about which we discussed"
Solution 1:
You don't need the about. However, I would probably also remove the which and add a the, making it:
I've added the changes/fixes we discussed yesterday.
Solution 2:
"Discuss" is transitive; you discuss a thing, you never discuss about a thing. As such, "about which we discussed" is simply incorrect.
Compare to "talk". You talk about a thing, you never "talk a thing". So the opposite would be true and you would have to say "the thing about which we talked", in this case.