Added parsing for '@1400000000 +0000' date format as used by git commit hooks by dries007 · Pull Request #965 · gitpython-developers/GitPython

The git pre-commit hook is run with the GIT_AUTHOR_DATE environment variable set.
The format used (on my machine anyway) is @1574863356 +0100, which is not handles correctly by parse_date.

This PR fixes that crash, but it doesn't quite fully resolve issue #963. This issue also contains steps to reproduce and a stacktrace.