fuse-cmd.py: flush the cache occasionally
fuse-cmd.py: update prefix when reconstructing cache parents

Previously, the cache reconstruction algorithm would fail whenever it
had to restore multiple parents when caching a given path.  Clearing
the cache revealed the problem.
fuse-cmd.py: use tuples for pre in _cache_get()

Avoid a bit of unnecessary work.
Merge work to occasionally clear the FUSE cache
Merge branch 'fix-fuse-mem'
change cache size from 100000 to 10000