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.

0 files changed, 0 insertions, 0 deletions