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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_upstream_fork_point(self):
 """Get the most recent ancestor of HEAD that occurs on an upstream branch. First looks at the current branch's tracking branch, if applicable. If that doesn't work, looks at every other branch to find the most recent ancestor of HEAD that occurs on a tracking branch. Returns: git.Commit object or None """

        possible_relatives = []
        try:
            if not self.repo:
                return None
            try:
                active_branch = self.repo.active_branch
            except (TypeError, ValueError):
                logger.debug("git is in a detached head state")
                return None  # detached head
            else:
                tracking_branch = active_branch.tracking_branch()
                if tracking_branch:
                    possible_relatives.append(tracking_branch.commit)

            if not possible_relatives:
                for branch in self.repo.branches:
                    tracking_branch = branch.tracking_branch()
                    if tracking_branch is not None:
                        possible_relatives.append(tracking_branch.commit)

            head = self.repo.head
            most_recent_ancestor = None
            for possible_relative in possible_relatives:
                # at most one:
                for ancestor in self.repo.merge_base(head, possible_relative):
                    if most_recent_ancestor is None:
                        most_recent_ancestor = ancestor
                    elif self.repo.is_ancestor(most_recent_ancestor, ancestor):
                        most_recent_ancestor = ancestor
            return most_recent_ancestor
        except exc.GitCommandError as e:
            logger.debug("git remote upstream fork point could not be found")
            logger.debug(e.message)
            return None