Typewriter: cache the committed state of a note in an ivar, used for computing the attributed string diff for the commit description, so we can avoid calling -objectGraphContextForPreviewingRevision: which is slow.

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10591 7b3f36aa-e6db-49c1-8b4c-66eac1997e64