This fixes spurious changes in timestamps.

Make MountState.write private.

Add a test for AutoUnionFs.

* getDataNoCache() -> GetFresh()

* getBranchResult() -> getBranch()

* struct getBranchResult -> branchResult

* Always strip trailing '/' from keys

* UnionFs.getBranchResult() to cut down on type casts.

* Introduct UnionFs.promoteDirsTo(), updating the branch cache for
  each parent directory too.

* On Chmod, update the branchcache too.

* On file release, drop the cache entry, which probably has incorrect
  file size.

This method gets called when a writable file gets closed in the
filesystem.  This allows filesystems to react to changed file sizes
due to writes.