Fix "git clone http://$URL" to check out the worktree when asked

The builtin-clone now does the http commit walking and the tree unpacking
in the same process, and the commit walker leaves the in-core objects in a
funny state.  When forgetting the data read from the tree object, the
object should be marked "not parsed yet" for later users.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

walker.c

@@ -59,6 +59,7 @@ static int process_tree(struct walker *walker, struct tree *tree)
 	free(tree->buffer);
 	tree->buffer = NULL;
 	tree->size = 0;
+	tree->object.parsed = 0;
 	return 0;
 }