Write the bitmap index correctly in DFS GC.

A bug caused the .bitmap to actually have the .idx contents.

Change-Id: I428bb27d419e8b1b69b6f3e2fd07cd29703669ad

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

@@ -339,7 +339,7 @@ public class DfsGarbageCollector {
 			out = objdb.writeFile(pack, BITMAP_INDEX);
 			try {
 				CountingOutputStream cnt = new CountingOutputStream(out);
-				pw.writeIndex(cnt);
+				pw.writeBitmapIndex(cnt);
 				pack.setFileSize(BITMAP_INDEX, cnt.getCount());
 			} finally {
 				out.close();