unused: Reduce memory usage significantly.

Much of the memory bloat turned out to be due to getKeysReferenced
containing a mapM, which is strict and buffered the whole list
rather than streaming it.

The other half of the bloat was due to building a temporary Set
in order to call S.difference. While that is more cpu efficient,
I switched to successive S.delete, since with it, I can run a whole
git annex unused in less than 8 mb of memory.

The whole Set of keys with content available is still stored in memory,
so running unused in a repo with a whole lot of file content will still
use more memory. In a repo containing 6000 files, it needed 40 mb.

Note that the status command still uses the bloatful getKeysReferenced.

doc/todo/git-annex_unused_eats_memory.mdwn

@@ -1,12 +1,20 @@
 `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
-50 mb when run in a repository with 80 thousand files.
+uses more memory than git-annex typically needs.
 
-(Used to be 80 mb, but implementation improved.)
+It used to be a lot worse (hundreds of megabytes).
 
-I would like to reduce this. One idea is to use a bloom filter. 
+Now it only needs enough memory to store a Set of all Keys that currently
+have content in the annex. On a lightly populated repository, it runs in
+quite low memory use (like 8 mb) even if the git repo has 100 thousand
+files. On a repository with lots of file contents, it will use more.
+
+Still, I would like to reduce this to a purely constant memory use,
+as running in constant memory no matter the repo size is a git-annex design
+goal.
+
+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 positives, this might