Not increasing the version when the tree is untouched

[b00f]

SaveVersion methods always increase the version of the tree.
I suggest to check the hash before increasing the version, if the hash has not changed, it can simply return the current hash. Something like:

if bytes.Compare(tree.LastHash, tree.WorkingHash()) == 0) // Not Hash()
  return tree.LastHash, tree.version, nil

Changing version will cause changing hash and it can cause application thinks that tree has changed which is not true.

UPDATE:
WorkingHash() (not Hash()) return current Hash.