Do not rewrite parents in RevWalkTextBuiltins

Default behavior of C git is to skip parent rewriting unless
particular history simplification or --parents flags are passed. Since
JGit has no such flags, JGit should not rewrite parents.

Change-Id: I9ba0e70fe6d5f49f975b71eea46f93198900f37d

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

@@ -211,11 +211,15 @@ abstract class RevWalkTextBuiltin extends TextBuiltin {
 	}
 
 	protected RevWalk createWalk() {
+		RevWalk result;
 		if (objects)
-			return new ObjectWalk(db);
-		if (argWalk != null)
-			return argWalk;
-		return argWalk = new RevWalk(db);
+			result = new ObjectWalk(db);
+		else if (argWalk != null)
+			result = argWalk;
+		else
+		  result = argWalk = new RevWalk(db);
+		result.setRewriteParents(false);
+		return result;
 	}
 
 	protected int walkLoop() throws Exception {