COUndoTrack: restore commit metadata added to synthesized commits made during undo/redo. This was broken during the last COUndoTrack rewrite. Add tests for the metadata.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10579 7b3f36aa-e6db-49c1-8b4c-66eac1997e64