Collapse version history when working with versions (VersionDiff, VersionRestore)

[janehmueller]

Some jobs write a Subject's field multiple times and thus write multiple versions to the version history. The jobs VersionDiff and VersionRestore both assume that each job only writes ones to a history field. Therefore all versions with the same version id have to be collapsed (e.g., always use the value of the last version that was written by a job) before they are used by the jobs.