Sounds like you described https://www.jetbrains.com/upsource/
(I am not affiliated with Jetbrains in any way)
(I am not affiliated with Jetbrains in any way)
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.