GitPython may raise a bad AssertionError

GitPython may raise an AssertionError during a successful fetch due to
poorly grepping the fetch output for success. If this is the case, a
second fetch will be a no-op. If the second fetch also raises the error,
then the error is legit.
Merge pull request #11519 from jesusaurus/develop

GitPython may raise a bad AssertionError