Bugfixes:
* dgit import-dsc: Do not crash with undefined $isuite. Closes:#850781.
+ * dgit build: Do not sometimes crash with undefined $isuite.
* dgit: Do not nedlessly re-fetch the rewrite map.
* dgit: After downloading .debian.* files, save them in `..', too
(ie do this not just for .origs).
+ * dgit: When fetching, refetch files with hash mismatches (and save them
+ as `...,fetch'), so we can distinguish them from any built locally.
+ Closes:#850824.
Test suite:
* Add test for import-dsc with default distro. (Detects #850781.)