vfs: cache resolve() calls to improve (fuse) performance

vfs: cache resolve() calls to improve (fuse) performance

Include resolve() results in the vfs cache.  This substantially
improves fuse "cat somefile" performance.  (Observed a ~2x rate
improvement with a 500MB urandom file).

This appears to be due to the fact that fuse read(path, offset, len)
is called many times for the file, resulting in many corresponding,
redundant resolve(path) calls.

The previous fuse implementation, based on the previous vfs had its
own cache, but moving the caching to the vfs should be more generally
helpful.

Now bup fuse will again ignore repository changes that affect paths it
has already examined.  This matches its behavior in the current stable
release (0.29.1).

Thanks to voldial for reporting the problem.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>