Add porcelain for diff-tree.
Add wrapper for commit-tree.
Add test for ConfigDict.iteritems.
Reimplement iteritems.
Add ConfigDict.itersections.
Update some copyright headers.
Move refs handling to a separate file.
Move read_info_refs from dulwich.client to dulwich.refs.
Move write_info_refs to dulwich.refs.
Move implementation of ObjectStore.get_graph_walker() to Repo.
Actually use Repo.get_parents.
Deprecate Repo.ref.
Fix test suite when run on systems without ~/.gitconfig.
Add configuration for tox.
Extend tox config to run without extensions.
Cope with functions not having func_name in pypy.
Fix docstring for StackedConfig.default_backends().

(Launchpad Bug 1252493)
Some minor improvements to _diff_tree.c:

- Simplify string handling
- Use PyErr_NoMemory()
Fix unnecessary incref of Py_False.
Graftpoint Tests

https://git.wiki.kernel.org/index.php/GraftPoint

GraftPoints are Commits that have their parents "rewritten"

To support graftpoints:

1) Repos must be able to manage its own graftpoints
2) Repos must be able to use graphwalkers that know how to use
graftpoints