DFS: Allow other RefDatabase implementations

Permit a DfsRepository implementation to use a different RefDatabase
than DfsRefDatabase.

Change-Id: Ia263285f547bde1943993cc994d0222185021a16

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

 import org.eclipse.jgit.lib.ObjectIdSet;
 import org.eclipse.jgit.lib.ProgressMonitor;
 import org.eclipse.jgit.lib.Ref;
+import org.eclipse.jgit.lib.RefDatabase;
 import org.eclipse.jgit.revwalk.RevWalk;
 import org.eclipse.jgit.storage.pack.PackConfig;
 import org.eclipse.jgit.storage.pack.PackStatistics;
 /** Repack and garbage collect a repository. */
 public class DfsGarbageCollector {
 	private final DfsRepository repo;
-
-	private final DfsRefDatabase refdb;
-
+	private final RefDatabase refdb;
 	private final DfsObjDatabase objdb;
 
 	private final List<DfsPackDescription> newPackDesc;
 
 		ctx = (DfsReader) objdb.newReader();
 		try {
-			refdb.clearCache();
+			refdb.refresh();
 			objdb.clearCache();
 
 			refsBefore = refdb.getRefs(ALL);

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

 		// Nothing to do.
 	}
 
+	@Override
+	public void refresh() {
+		clearCache();
+	}
+
 	@Override
 	public void close() {
 		clearCache();

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

 	@Override
 	public abstract DfsObjDatabase getObjectDatabase();
 
-	@Override
-	public abstract DfsRefDatabase getRefDatabase();
-
 	/** @return a description of this repository. */
 	public DfsRepositoryDescription getDescription() {
 		return description;
 	 *             the repository cannot be checked.
 	 */
 	public boolean exists() throws IOException {
-		return getRefDatabase().exists();
+		if (getRefDatabase() instanceof DfsRefDatabase) {
+			return ((DfsRefDatabase) getRefDatabase()).exists();
+		}
+		return true;
 	}
 
 	@Override
 
 	@Override
 	public void scanForRepoChanges() throws IOException {
-		getRefDatabase().clearCache();
+		getRefDatabase().refresh();
 		getObjectDatabase().clearCache();
 	}