Instead of populating the second-level Bloom filter with every key
referenced in every Git reference, consider only those which differ
from what's referenced in the index.
Incidentaly, unlike with its old behavior, staged
modifications/deletion/... will now be detected by 'unused'.
Credits to joeyh for the algorithm. :-)
Making the pre-commit hook look at git diff-index to find changed direct
mode files and update the mappings works pretty well.
One case where it does not work is when a file is git annex added, and then
git rmed, and then this is committed. That's a no-op commit, so the hook
probably doesn't even run, and it certianly never notices that the file
was deleted, so the mapping will still have the original filename in it.
For this and other reasons, it's important that the mappings still be
treated as possibly inconsistent.
Also, the assistant now allows the pre-commit hook to run when in direct
mode, so the mappings also get updated there.
