I hacked on this a bit and came up with the attached files, which should be 
improvements.  'announcement.txt' is a draft email to be sent to emacs-devel 
about a week before the change.  The two patch files have the same role as 
before, and attempt to address the comments made in this thread.

It's now much easier to edit ChangeLog history: run 'make change-history' to 
prepend newer commit messages to the file ChangeLog.1 (thus marking them as 
older), edit ChangeLog.1 by hand, and then commit the result.  Once ChangeLog.1 
gets too big it should be easy to start working on ChangeLog.2.

I had trouble implementing Glenn's suggestion of using a '#' prefix to mark 
ignorable commit messages, as Git typically uses '#' for its own purposes when 
editing commit messages.  I changed the prefix to '; ' and this worked better, 
so I went with that.  It should be easy to use any string that starts with a 
character other than '#'.