patches-applied historical imports (usd import)

[peter green]

> I haven't run dgit's dsc importer on a whole historical archive but
> Peter Green of Raspian has been running it and filing bugs.  I haven't
> seen such a bug recently so I hope it has been working for all the
> packages he's seen.
I haven't really been doing large scale importing, I have just imported relatively shallow histories of a relative handful of packages.