Fix two bugs in COSynchronizer. First, it was using a poor choice of common ancestor when rebasing commits (added a comment explaining the new choice, _lastRevisionUUIDInTransitToServer). Second, the selection of commits to merge in order to generate the rebased revisions was wrong, and led to changes being added more than once

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10082 7b3f36aa-e6db-49c1-8b4c-66eac1997e64