Faster checkout times under Git

[Don Stewart]

Eric Mertens kindly did some experiments on the various git repos,
and servers, and approaches to serving.

* We're looking at >45 mins for a full history darcs get
  of ghc, over http, from darcs.haskell.org.

* git clone of full ghc over http, from darcs.haskell.org, completes in
  the range of 6-7 minutes (roughly 150KiB/s)

* git clone over git protocol, using github's bandwidth, completes
  in 2.1 minutes. (roughly 560KiB/s)

So that indicates a significant improvment by switching to the git://
server protocol. Can we get that on darcs.haskell.org?

In general git is doing a good job here addressing slow 'darcs get '
times, which are now way way down. This will make life easier for some
of us.

Mirroring automatically to github could also address some of our
redundancy concerns.

-- Don