-[COSynchronizerUtils rebaseRevision:...]: avoid recomputing one of the two diffs. Reduces the time of -testAttributedStringPerformance by 33%

git-svn-id: http://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@10105 7b3f36aa-e6db-49c1-8b4c-66eac1997e64