+ * Introduce --rm-old-changes to delete previous builds' changes files.
+ * Remove any pre-existing _source.changes file before building source,
+ as a safety check.
+ * No longer tolerate a multitude of .changes files when doing push.
+ Instead, insist on a single one.
+ * dgit sbuild no longer deletes extranious .changes files; instead
+ we rely on --rm-old-changes, or failing that, fail early.