Update pygit2 to 0.28.0

[pyup-bot]

This PR updates pygit2 from 0.27.4 to 0.28.0.

Changelog

0.28.0

-------------------------

- Upgrade to libgit2 0.28
`878 <https://github.com/libgit2/pygit2/issues/878>`_

- Add binary wheels for Linux
`793 <https://github.com/libgit2/pygit2/issues/793>`_
`869 <https://github.com/libgit2/pygit2/pull/869>`_
`874 <https://github.com/libgit2/pygit2/pull/874>`_
`875 <https://github.com/libgit2/pygit2/pull/875>`_
`883 <https://github.com/libgit2/pygit2/pull/883>`_

- New ``pygit2.Mailmap``, see documentation
`804 <https://github.com/libgit2/pygit2/pull/804>`_

- New ``Repository.apply(...)`` wraps ``git_apply(..)``
`841 <https://github.com/libgit2/pygit2/issues/841>`_
`843 <https://github.com/libgit2/pygit2/pull/843>`_

- Now ``Repository.merge_analysis(...)`` accepts an optional reference parameter
`888 <https://github.com/libgit2/pygit2/pull/888>`_
`891 <https://github.com/libgit2/pygit2/pull/891>`_

- Now ``Repository.add_worktree(...)`` accepts an optional reference parameter
`814 <https://github.com/libgit2/pygit2/issues/814>`_
`889 <https://github.com/libgit2/pygit2/pull/889>`_

- Now it's possible to set SSL certificate locations
`876 <https://github.com/libgit2/pygit2/issues/876>`_
`879 <https://github.com/libgit2/pygit2/pull/879>`_
`884 <https://github.com/libgit2/pygit2/pull/884>`_
`886 <https://github.com/libgit2/pygit2/pull/886>`_

- Test and documentation improvements
`873 <https://github.com/libgit2/pygit2/pull/873>`_
`887 <https://github.com/libgit2/pygit2/pull/887>`_

Breaking changes:

- Now ``worktree.path`` returns the path to the worktree directory, not to the
`.git` file within
`803 <https://github.com/libgit2/pygit2/issues/803>`_

- Remove undocumented ``worktree.git_path``
`803 <https://github.com/libgit2/pygit2/issues/803>`_

Links

• PyPI: https://pypi.org/project/pygit2
• Changelog: https://pyup.io/changelogs/pygit2/
• Repo: https://github.com/libgit2/pygit2

[EnTeQuAk]

This needs an update of libgit2 in scripts/travis-install-libgit2.sh

[EnTeQuAk]

Looking at this more closely, we may run into issues with the debian versions too. https://packages.debian.org/search?searchon=names&keywords=libgit2

I'll test this locally and see how this works out.

[EnTeQuAk]

Trying to make use of upstream provided wheel packages, it looks promising locally. Let's see