reference object equality

[imbuedhope]

Similar to #852

References don't seem to have any sense of equality so the following happens

>>> import pygit2
>>> repo = pygit2.Repository('.')
>>> repo.references['refs/heads/master'] == repo.references['refs/heads/master']
False

This breaks container operations with Reference objects.

Unlike objects, references are not necessarily immutable since they can be force pushed to point to a different target.

Implementing __eq__ to be based on Reference.name makes the most sense, I think. __hash__ support does not make sense since references are mutable.

[jdavid]

+1