Add git changelog method.
Add git changelog method.

Change-Id: I71d99f00b2154bf151a1521016118ba6bd088f79

citrix-openstack-build/pbr,citrix-openstack-build/pbr,openstack-dev/pbr,WiFast/pbr,openstack-dev/pbr,klmitch/pbr,WiFast/pbr,JioCloud/pbr,varunarya10/pbr,citrix-openstack-build/pbr,klmitch/pbr,redhat-openstack/pbr,emonty/oslo-packaging,emonty/oslo-packaging,redhat-openstack/pbr,varunarya10/pbr,JioCloud/pbr,openstack-dev/pbr