Change to extensions.refsStorage

git-core just rerolled the extensible backends series with refsStorage
as the configuration key. Update JGit to match git-core.

Change-Id: If345a2403a996e358b29cfa2a2298f6e8d59d96b

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/FileRepository.java

@@ -207,7 +207,7 @@ public class FileRepository extends Repository {
 				ConfigConstants.CONFIG_KEY_REPO_FORMAT_VERSION, 0);
 
 		String reftype = repoConfig.getString(
-				"extensions", null, "refsBackendType"); //$NON-NLS-1$ //$NON-NLS-2$
+				"extensions", null, "refsStorage"); //$NON-NLS-1$ //$NON-NLS-2$
 		if (repositoryFormatVersion >= 1 && reftype != null) {
 			if (StringUtils.equalsIgnoreCase(reftype, "reftree")) { //$NON-NLS-1$
 				refs = new RefTreeDatabase(this, new RefDirectory(this));