Install git with source-repositories element.

Git isn't a dependency of this element, but is a dependency of pbr
if you are doing pip install on a git cloned directoy, because this
looks to be a fairly common pattern it makes sense to have git on the
target image.

Change-Id: I49ee3fa4eac493b9dfa379a674ab9b3390cfc89a
Merge "Install git with source-repositories element."