Update ChangeLog with 1.2.0 release date

git-svn-id: http://svn.savannah.nongnu.org/svn/rdiff-backup@919 2b77aa54-bcbc-44c9-a7ec-4f6cf2b41109

trunk/rdiff-backup/CHANGELOG

-New in v1.1.18 (????/??/??)
+New in v1.2.0 (2008/07/27)
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 Fall back on the Python make_file_dict function when the filename contains