R1:e08ba99dd3db

Proxy the "git upload-pack" wire protocol

Summary:
Depends on D20380. Ref T8093. When prototypes are enabled, inject a (hopefully?) no-op proxy into the Git wire protocol.

This proxy decodes "git upload-pack" and allows the list of references to be rewritten, in a similar way to how we already proxy the Subversion protocol to rewrite URIs and proxy the Mercurial protocol to distinguish between read and write operations.

The piece we care about comes at the beginning, and looks like this:

```
<frame…