Refactors credentials callback on remote to make pushes authenticate

[eldavido]

#437

There's a single test break in this:

======================================================================
FAIL: test_remote_refcount (test.test_remote.RepositoryTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/dalbrecht/pygit2/test/test_remote.py", line 189, in test_remote_refcount
    self.assertEqual(start, end)
AssertionError: 2 != 3

----------------------------------------------------------------------
Ran 228 tests in 9.560s

FAILED (failures=1)

It looks like the patch is causing the refcount to get thrown. If someone could take a look at why this is happening, we can get this merged and pushes should work with credentials.

[carlosmn]

The reason is that you removed a self._self_handle = None assignment, which is necessary to break the cyclic reference which python cannot break itself. That is what's keeping #431 from getting merged, which attempts to solve this problem as well.

[jdavid]

So issue #437 and PRs #431 and #438 are about the same problem? Would be nice to get this fixed for the upcoming release.

[carlosmn]

Yeah, I was hoping @DrWong would get back to us on the handle issue, but I can squash the fix in and push that to master if you'd like. We can still have a refactoring after the pushing bit is fixed.

[jdavid]

@carlosmn that would be cool

[jdavid]

So I understand this is fixed.. closing