we rely on --rm-old-changes, or failing that, fail early.
* When doing quilt linearisation, treat upstream .gitignores not
in the toplevel the same way we treat ones in the toplevel.
+ * When automatically generating quilt patch, honour GIT_COMMITTER_DATE
+ for filename creation (makes filename deterministic in test suite).
Infrastructure:
* Better error handling in dgit-repos-policy-debian.