Fix jgit rev-list --objects master

This flag was not being honored due to a bug in createWalk().
argWalk is always non-null when there are commits passed in
on the command line. If --objects was specified, always make
a new ObjectWalk for the actual execution.

Change-Id: I6e1a1636f2634605d86671a83766cc1c42939821
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/RevWalkTextBuiltin.java

@@ -199,9 +199,11 @@ abstract class RevWalkTextBuiltin extends TextBuiltin {
 	}
 
 	protected RevWalk createWalk() {
-		if (argWalk == null)
-			argWalk = objects ? new ObjectWalk(db) : new RevWalk(db);
-		return argWalk;
+		if (objects)
+			return new ObjectWalk(db);
+		if (argWalk != null)
+			return argWalk;
+		return new RevWalk(db);
 	}
 
 	protected int walkLoop() throws Exception {