Make BitmapBuilder.getBitmapIndex public

Every Bitmap in current JGit code has an associated BitmapIndex.  Make
it public in BitmapBuilder to make retrieving bitmaps to OR in from
that index easier.

Change-Id: I2773aa94d8b67f12194608e6317c0792a5de21e2

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

@@ -318,7 +318,8 @@ public class BitmapIndexImpl implements BitmapIndex {
 			return true;
 		}
 
-		BitmapIndexImpl getBitmapIndex() {
+		@Override
+		public BitmapIndexImpl getBitmapIndex() {
 			return BitmapIndexImpl.this;
 		}
 	}

org.eclipse.jgit/src/org/eclipse/jgit/lib/BitmapIndex.java

@@ -204,5 +204,12 @@ public interface BitmapIndex {
 
 		/** @return the number of elements in the bitmap. */
 		int cardinality();
+
+		/**
+		 * The BitmapIndex for this BitmapBuilder.
+		 *
+		 * @since 4.2
+		 */
+		BitmapIndex getBitmapIndex();
 	}
 }