git fetch: Monkey patch GitPython bug

There is an assertion inside of GitPython that is causing false
negatives, so the plugin occasionally fails without any reason.

This patch simply filters out the false negative exception. It's not an
optimal solution, since it also filters out the right occurences of
failed assertion.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>
Finished feature 54182062/invalid-assertion.