COUndoTrack: set a commit descriptor on the commit that does a selective undo of a track node. This gives a nice description in the projectdemo undo window like 'Selective undo 'foo''

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10062 7b3f36aa-e6db-49c1-8b4c-66eac1997e64