add hash and equality operations on git objects

[imbuedhope]

Resolves issue #852 by implementing __eq__, __ne__, and __hash__ for git _pygit.Object types. Uses contained Oid as a unique identifier and falls back to the Oid operations for the same.

The net result is that the following now happens without issue

>>> import pygit2
>>> repo = pygit2.Repository('.')
>>> repo.head.get_object() == repo.head.get_object()
True
>>> repo.head.get_object() != repo.head.get_object()
False

Note that unlike Oid __ge__, __gt__, __le__, and __lt__ are not supported. These comparisions don't really make sense for Object.

As a bonus Object and its subtypes can be used in with set and other hash depenent containers. For example,

>>> import pygit2
>>> repo = pygit2.Repository('.')
>>> s = set()
>>> s.add(repo.head.get_object())
>>> s.add(repo.head.get_object())
>>> len(s)
1

Note that following is also true, but is probably not a gurantee since a later imlpementation may break it.

>>> import pygit2
>>> repo = pygit2.Repository('.')
>>> hash(repo.head.get_object()) == hash(repo.head.get_object().oid)
True

[jdavid]

Thanks!

It would be more efficient to use git_object_id and git_oid_equal. Please add unit tests as well.

[imbuedhope]

Reimplemented Object_richcompare and Object_hash to be a bit more efficient and added tests in test/test_object.py.