Changed _get_git_directory() to use git itself.

This lets pbr work on projects that are checked out as submodules.

Change-Id: I8c47a7dbae0bd390bd2529f972ffe9920aed374e
Reviewed-on: https://review.openstack.org/29619
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins