Reviewed by darin.

        http://bugzilla.opendarwin.org/show_bug.cgi?id=9395
        Make prepare-ChangeLog faster

        * Scripts/prepare-ChangeLog: Use "svn diff" instead of "svn stat" to find changed files,
        then save the diff output for reuse.  Keep a status variable if changes are made to
        LayoutTests so "svn diff LayoutTests" doesn't have to be run to check for changes when
        WebCore is updated.  Added -h|--help command-line switch and help message.  Move test
        for no changed files closer to the beginning of the program.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@14814 268f45cc-cd09-0410-ab3c-d52691b4dbfc