R1:c7f23f522a79

Accept and route VCS HTTP requests

Summary:
Mostly ripped from D7391, with some changes:

- Serve repositories at `/diffusion/X/`, with no special `/git/` or `/serve/` URI component.
- This requires a little bit of magic, but I got the magic working for Git, Mercurial and SVN, and it seems reasonable.
- I think having one URI for everything will make it easier for users to understand.
- One downside is that git will clone into `X` by default, but I think that's not a big deal, and we can…