changelog: don't cluster multiple entries under the same "date line"

changelog: don't cluster multiple entries under the same "date line"

* lib/gitlog-to-changelog: Synced from gnulib.  The new version
has a new option '--no-cluster', that disables clustering of
adjacent commit messages under the same "date line".
* Makefile.am (gitlog_to_changelog_options): Add '--no-cluster'.
Also add a proper '--format' specification to ensure we have a
blank line between the summary line and the commit message body.