dev/sg: migrate sg generate to use sg/run, render nice diff, do not lint generate in dirty dir

Created by: bobheadxi

With uncommitted changes, sg lint go is inaccurate because it reports all your code changes as generated changes. Maybe we can do some kind of stashing first, but that will affect other lints, so let's just do this for now.

Tangential effort trying to see where we might be losing output from https://sourcegraph.slack.com/archives/C01N83PS4TU/p1652912619242789

Test plan