WIP: Do HTTP like core Mercurial
This changes the HTTP support to intercept Dulwich requests and layer them through urllib2 & Mercurial classes when using Dulwich 0.20.24 or later.
The upside is that it means that cloning Git repositories over HTTP uses the exact same settings as regular Mercurial HTTP repositories. Unfortunately, that includes not supporting ~/.git-credentials
files.
Also, it kinda-sorta breaks the test suite, due to different password prompts. Hence, the draft status.