Minor refactoring to the diff code. Add a test case for diff.h. Add a 'copy' edit that indicates matching ranges in the diff, which may be useful for implementing the attributed string diff. Add some interfaces for the attributed string diff class.

git-svn-id: svn://svn.gna.org/svn/etoile/trunk/Etoile/Frameworks/CoreObject@9922 7b3f36aa-e6db-49c1-8b4c-66eac1997e64