This is needed because when preferred content matches on files,
the second pass would otherwise want to drop all keys. Using a bloom filter
avoids this, and in the case of a false positive, a key will be left
undropped that preferred content would allow dropping. Chances of that
happening are a mere 1 in 1 million.
This makes git annex unused use around 48 mb more memory than it did before,
but the massive increase in accuracy makes this worthwhile for all but the
smallest systems.
Also, I want to use the bloom filter for sync --all --content, to avoid
dropping files that the preferred content doesn't want, and 1/1000
false positives would be far too many in that use case, even if it were
acceptable for unused.
Actual memory use numbers:
1000: 21.06user 3.42system 0:26.40elapsed 92%CPU (0avgtext+0avgdata 501552maxresident)k
1000000: 21.41user 3.55system 0:26.84elapsed 93%CPU (0avgtext+0avgdata 549496maxresident)k
10000000: 21.84user 3.52system 0:27.89elapsed 90%CPU (0avgtext+0avgdata 549920maxresident)k
Based on these numbers, 10 million seemed a better pick than 1 million.
