Earlier quoted context omitted.
Sounds like you described https://www.jetbrains.com/upsource/ (I am not affiliated with Jetbrains in any way)
I've seen a couple brief comments about Upsource in this thread. Can anyone give a deeper dive on their experiences? I tried it very briefly but didn't have time to configure integration with my IDE, just played in the web browser. I suspect that means I didn't try the best feature. Pros/cons of this tool?
Pros: I'm obviously biased so I won't go into those.
Cons: it's not a lightweight tool that's happy with 1 gig of memory. Configuring and waiting for code indexes takes a bit of time, especially if you're trying it out on large legacy codebases. It also won't manage a Git repository for you.