Update gitpython to 2.1.13

[pyup-bot]

This PR updates GitPython from 2.1.11 to 2.1.13.

Changelog

2.1.12

==============================

* Multi-value support and interface improvements for Git configuration. Thanks to A. Jesse Jiryu Davis.

see the following for (most) details:
https://github.com/gitpython-developers/gitpython/milestone/27?closed=1

or run have a look at the difference between tags v2.1.11 and v2.1.12:
https://github.com/gitpython-developers/GitPython/compare/2.1.11...2.1.12

Links

• PyPI: https://pypi.org/project/gitpython
• Changelog: https://pyup.io/changelogs/gitpython/
• Repo: https://github.com/gitpython-developers/GitPython
• Docs: https://pythonhosted.org/GitPython/