HAMMER Utilities: New utility 'undo'.

HAMMER Utilities: New utility 'undo'.

* Add a new utility called 'undo' which makes use of HAMMER capabilities
  to retrieve prior versions of a file, even if that file has been deleted.

  This utility can dump all prior versions of the file, generate a history
  of transaction ids associated with the file, has a 'quick diff' relative
  to the most recent change, and can also generate diffs for all versions
  of the file.

  This utility only works with HAMMER filesystems.