DFS: Override exactRef() with optimal version

This avoids scanning a directory to find all names and
then weeding down to the requested name.

Change-Id: I36ce31febad6e881182bc3a278c7ed35cc04cbda

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/dfs/DfsRefDatabase.java

@@ -90,6 +90,13 @@ public abstract class DfsRefDatabase extends RefDatabase {
 		return 0 < read().size();
 	}
 
+	@Override
+	public Ref exactRef(String name) throws IOException {
+		RefCache curr = read();
+		Ref ref = curr.ids.get(name);
+		return ref != null ? resolve(ref, 0, curr.ids) : null;
+	}
+
 	@Override
 	public Ref getRef(String needle) throws IOException {
 		RefCache curr = read();