libgit2/pygit2 — pull request #96 by cholin (2012-05-30 06:05:01+00:00)
Title: support for diff against index or workdir

Added missing support for diff against an index or current working dir.

example
```
from pygit2 import Repository

repo = Repository('/tmp/dirtyrepo')
head = repo[repo.lookup_reference('HEAD').resolve().oid]
tree = head.tree
diff_index = tree.diff(repo.index)
diff_work = tree.diff()

print(diff_index.changes)
print(diff_work.patch)
```