Ensure /usr/ports is a Git repository on FreeBSD

Summary:
The current recommended way to manage the /usr/ports directory is
to use it as a Git repository.

As /usr/ports can or can't be already exist, managed by portsnap,
or just installed by the installation process, this script takes
care of handle all the case by creating if needed the directory,
then setting up Git inside.

Ref T2019

Test Plan: Deploy on new WindRiver server

Reviewers: dereckson, DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T2019

Differential Revision: https://devcentral.nasqueron.org/D3444