Remove CommitDir, and avoid loading all commits at startup.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
fuse-cmd.py: ignore metadata for top *two* levels

Now that .commit is gone, stop ignoring the filesystem root dir
metadata (which was also at level 3).
Notice new saves from FUSE and the VFS
Change fuse purge threshold from 100k to 10k