Add class to encapsulate collections

We need this, because we have different trees of RefTreeNode objects
just below the Repo and Namespace nodes. We could make refs/tags/some/foo
and refs/heads/some/bar both be children of a single RefTreeNode("some")
below the Repo. But that doesn't sound like any nice solution.
Creation of RefTreeNodes
Dump refname
Refresh Repo on creation
Don't delete children twice
Dump Repo-Name if available