`git-annex unused` has to compare large sets of data
(all keys with content present in the repository,
with all keys used by files in the repository), and so
uses more memory than git-annex typically needs; around
60-80 mb when run in a repository with 80 thousand files.

I would like to reduce this. One idea is to use a bloom filter. 
For example, construct a bloom filter of all keys used by files in
the repository. Then for each key with content present, check if it's
in the bloom filter. Since there can be false negatives, this might
miss finding some unused keys. The probability/size of filter
could be tunable.

Another way might be to scan the git log for files that got removed
or changed what key they pointed to. Correlate with keys with content
currently present in the repository (possibly using a bloom filter again),
and that would yield a shortlist of keys that are probably not used. 
Then scan thru all files in the repo to make sure that none point to keys
on the shortlist.