Fix version number comment
MutableSet (extends Set) and PersistentSet interfaces
Remove VersionGraph.lock

If locking is required is should be done at higher level.
Remove VersionGraph.parentGraph - use PersistentTreeMap for versionNodes
Builder.versions -> versionNodes
Track VersionGraph leaves. Remove VersionNode.previous.
Heads ordered by branch and revision for easy access to branch heads.
Merge branches. PersistentSortedMap.range(from, to).