This commit removes the following:
* This script's dependency on gitpython, it is not maintained actively.
* The ability to push changes in this and the main zulip/ repo. Doing
so automatically was turning out to be cumbersome and buggy and
doing it manually doesn't take significantly longer anyway.
* The ability to automatically increment PROVISION_VERSION in
zulip/version.py. Again, this was too buggy and it doesn't take
much longer to just increment it by hand.
Removing the above features made the script a lot easier to maintain
and read.
3c23dd6c66