Migrate COLeastCommonAncestor code to use the revision cache. This fixes the bottleneck in Typewriter when redoing ~50 undo commands at once

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10348 7b3f36aa-e6db-49c1-8b4c-66eac1997e64