This involve introducing new Consistency states `STAGED` and `BOTH` that
represent when the source is just in the local CAS and in both the local
CAS and unstaged in the source directory.
Sources are staged for each element into the local CAS during the fetch
stage. If the sources are in the local consistency state `STAGED` when
wanting to open a workspace, the original sources are fetched.
Relavant tests this affects have been changed.
Part of #440