Reduce BitmappedObjectReachabilityChecker visibility

ObjectReachabilityChecker interface is the only public API. The
implementation is instantiated by ObjectWalk and doesn't need to be
visible outside the package.

Change-Id: I5b97bb98990cded637686bdc15c9655330b7780f
Signed-off-by: Ivan Frade <ifrade@google.com>

org.eclipse.jgit/src/org/eclipse/jgit/revwalk/BitmappedObjectReachabilityChecker.java

@@ -25,10 +25,8 @@ import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
 /**
  * Checks if all objects are reachable from certain starting points using
  * bitmaps.
- *
- * @since 5.8
  */
-public class BitmappedObjectReachabilityChecker
+class BitmappedObjectReachabilityChecker
 		implements ObjectReachabilityChecker {
 
 	private final ObjectWalk walk;