i'm glad someone who works at the company like you was able to enlighten us as to microsoft's internal practices
oh wait
oh wait
The only difference is that you can see the source and test on your own machine first.
MS employees don't have to commit to any codebase they like, but they should be able to obtain the actual source, so that "with enough eyes, all bugs are shallow".
That's pretty much how it works at Google. The code owners have to review and approve your change (and can reject it), but it's common (even encouraged) to commit a bugfix or new feature to someone else's project.
PS: While the linked article is very relevant here, I'm getting tired of people always linking to Joel's blog -- he's not the definitive software spokesperson; has strong opinions which have been shaped by years at Microsoft - and a lot of his opinions are boxed into those views.