Don't use GitPython for authors check
Don't use GitPython for authors check

I hit this issue with an older GitPython version:

  https://bugzilla.redhat.com/740852

Seeing as all GitPython is doing for us is running:

  $> git rev-list --pretty=raw HEAD

and parsing the output, I figure it's just as easy for us to run git
directly.

Change-Id: Ib43e2346c7d4c47e7776919f2b9c982133628129

openstack-dev/hacking,zancas/hacking,zancas/hacking,hyakuhei/cleantox,emonty/oslo-hacking,hyakuhei/cleantox,emonty/oslo-hacking,openstack-dev/hacking