Fix assertion failure when saving checkpoint revision

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10515 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
CoreObject: add 'checkpoint' commit descriptor

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10516 7b3f36aa-e6db-49c1-8b4c-66eac1997e64
Typewriter: add a save command that does a checkpoint commit

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10517 7b3f36aa-e6db-49c1-8b4c-66eac1997e64