FreeBSD port for GitPython now uses lowercase

Summary:
Salt recommends and uses pygit2, bindings for libgit2,
but staging-commit-message.py still uses that one.

Test Plan: salt-call --local state.sls roles/salt-primary/software

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2643