Store the old kernel changelog entries in kernel-docs package (bsc#1218713)
    
    The old entries are found in kernel-docs/old_changelog.txt in docdir.
    
    rpm/old_changelog.txt can be an optional file that stores the similar
    info like rpm/kernel-sources.changes.old.  It can specify the commit
    range that have been truncated.  scripts/tar-up.sh expands from the
    git log accordingly.