* 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.
+ Instead, insist on a single one. Closes:#800110.
* dgit sbuild no longer deletes extranious .changes files; instead
we rely on --rm-old-changes, or failing that, fail early.
* When doing quilt linearisation, treat upstream .gitignores not