history window: don't disable selective undo buttons when sharing

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10636 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
Fixed weak reference crash on 10.7

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10637 7b3f36aa-e6db-49c1-8b4c-66eac1997e64