Specialized Map to present a {@code RefDatabase} namespace.
Although not declared as a {@link java.util.SortedMap}, iterators from this map's projections always return references in {@link RefComparator} ordering.The map's internal representation is a sorted array of {@link Ref} objects,which means lookup and replacement is O(log N), while insertion and removal can be as expensive as O(N + log N) while the list expands or contracts. Since this is not a general map implementation, all entries must be keyed by the reference name.
This class is really intended as a helper for {@code RefDatabase}, which needs to perform a merge-join of three sorted {@link RefList}s in order to present the unified namespace of the packed-refs file, the loose refs/ directory tree, and the resolved form of any symbolic references.