Remov final keyword to the WalkGraphOp class or it can't be enhanced at runtime
Add --orphan option to cli's Branch command, the porcelain command had it already
Fix storage order of tree nodes when trees are split.

This change breaks compatibility with repositories created before
this change because it affects (and fixes) the order in which
nodes are assigned to buckets when trees are split into subtrees.

Now NodePathStorageOrder uses the absolute value of the FNV-1a hash
used to set the node order.
Merge pull request #556 from groldan/issue_551

Fix storage order of tree nodes when trees are split.