Log Message:
py-pygit2: update to 0.28.2.

0.28.2 (2019-05-26)
-------------------------

- Fix crash in reflog iteration
  `#901 <https://github.com/libgit2/pygit2/issues/901>`_

- Support symbolic references in ``branches.with_commit(..)``
  `#910 <https://github.com/libgit2/pygit2/issues/910>`_

- Documentation updates
  `#909 <https://github.com/libgit2/pygit2/pull/909>`_

- Test updates
  `#911 <https://github.com/libgit2/pygit2/pull/911>`_