ObjectReachabilityCheckers: Make walk member final

It is only assigned on initialization.

Change-Id: I36f68b3f511236e66cc7a4f56ba49252b317276a
Signed-off-by: Ivan Frade <ifrade@google.com>

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

@@ -29,7 +29,7 @@ import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
 public class BitmappedObjectReachabilityChecker
 		implements ObjectReachabilityChecker {
 
-	private ObjectWalk walk;
+	private final ObjectWalk walk;
 
 	/**
 	 * New instance of the reachability checker using a existing walk.

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

@@ -23,7 +23,7 @@ import org.eclipse.jgit.errors.MissingObjectException;
  * walk.
  */
 class PedestrianObjectReachabilityChecker implements ObjectReachabilityChecker {
-	private ObjectWalk walk;
+	private final ObjectWalk walk;
 
 	/**
 	 * New instance of the reachability checker using a existing walk.