Openstack Swift backend support for bare repositories

This patch adds a new SwiftRepo object that inherits of
BaseRepo. The SwiftRepo object knows how to deal with
git repositories stored on an Openstack object storage
Swift.

Regular filesystems are the defacto backend for Git
repositories but suffers of some limitations in term of
availability and scalability. Using Swift as backend, Git
objects are duplicated in multiple replica, that improves a lot
the disponibity of your repositories, avoids the downtime
due to infrastructure maintenance and also avoids the need to
configure backups. Extending a regular filesystem where
your repositories are stored can be risky and can leads to
downtime. With Swift as backend, extending the storage size
is transparent.

For more details have a look to the README.swift file
provided by this patch.
Move the swift backend to dulwich/contrib.

Subcommands for initializing repositories on swift are now available
from the dulwich.contrib.swift module. In the future, we could
expose this functionality as a dulwich-swift script. The main reason
I haven't done this yet is to keep it clear that this script is in
contrib.
Fix logic for extra adds of identical files.

Additionally, add a test.
Update NEWS.