Allow set-source-vars to consume a git manifest

This patch allows you to use git manifest files to generate
all the "export DIB_REPO...." commands you would need to be
able to reproduce a build using the same SHA1s as a previous
build.

Production of git manifests by DIB was added in
https://review.openstack.org/#/c/75829/

Change-Id: Ie72ad9697bf790cf13f2335c6290fb1d8ab599fb
Merge "Allow set-source-vars to consume a git manifest"