Add VersionedManifests

VersionedManifests extends VersionedMetaData so that manifests in a git
commit can be retrieved and saved.

org.eclipse.jgit.junit is added at a version that is supported in Gerrit
2.9.1 so that a mock git repo/commit can be created for tests.

Change-Id: I15cc5bea49e32aa99169dfdffab663b9f21c52be