I spent a lot of time working on JGit: http://eclipse.org/jgit/
This is going to take folks a long time to do in pure JS... before even work has to be done to tweak performance... the great thing about JGit is that it's under EDL (BSD) so JS-Git should be able to steal some ideas
It shows a lot of promise. Coming from someone who basically despises adobe this platform has given them an ounce of respect from me. Very interesting ideas, a web 2.0 take on ides in general and aims at being a go to for all those engineers turned web developer. Im not sure if it was their idea or they copied it but cross file editing of css styles inline is especially cool. As is the fact that you can hack on the source during runtime.