- move to file-per-changelog for unreleased change notes,

- move to file-per-changelog for unreleased change notes,
so that we no longer have to rely upon merges within
the changelog files.   because gerrit doesn't allow us
to use custom merge engines unlike git, we have no ability
to merge multiple changes into the changelog files without
going through conflicts.   new version of changelog
in git supports these new patterns.

Change-Id: I588eebd027b4ad3428bd522f36a6f1cd77f105bb
(cherry picked from commit 39c72682b85c1d3912ced383733d03eca7a43845)