use --diff=none for debmirror

Diffs are slow and consist of many files. Our server bandwidth is high
and we infrequently update, so don’t use diffs at all.
compute-ranking: use binary-amd64/Packages.bz2 instead of apt-cache

This makes the ranking computation _a lot_ faster (it now runs in ≈ 15s
wall clock time).

More importantly, it drops the dependency on an up-to-date apt index.

closes #6