Added Git.ByteString which replaces Git IO methods with ones using lazy
ByteStrings. This can be more efficient when large quantities of data are
being read from git.
In Git.LsTree, parse git ls-tree output more efficiently, thanks
to ByteString. This benchmarks 25% faster, in a benchmark that includes
(probably predominately) the run time for git ls-tree itself.
In real world numbers, this makes git annex unused 2 seconds faster for
each branch it needs to check, in my usual large repo.
Fixed the laziness space leak, so it runs in 60 mb or so again. Slightly
faster due to using Data.Set.difference now, although this also makes it
use slightly more memory.
Also added display of the refs being checked, and made unused --from
also check all refs for things in the remote.
