mark_parents_uninteresting(): replace list with stack

The mark_parents_uninteresting() function uses two loops:
an outer one to process our queue of pending parents, and an
inner one to process first-parent chains. This is a clever
optimization from 941ba8db57 (Eliminate recursion in
setting/clearing marks in commit list, 2012-01-14) to limit
the number of linked-list allocations when following
single-parent chains.

Unfortunately, this makes the result a little hard to read.
Let's replace the list with a stack. Then we don't have to
worry about doing this double-loop optimization, as we'll
just reuse the top element of the stack as we pop/push.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -91,20 +91,42 @@ void mark_tree_uninteresting(struct tree *tree)
 	mark_tree_contents_uninteresting(tree);
 }
 
+struct commit_stack {
+	struct commit **items;
+	size_t nr, alloc;
+};
+#define COMMIT_STACK_INIT { NULL, 0, 0 }
+
+static void commit_stack_push(struct commit_stack *stack, struct commit *commit)
+{
+	ALLOC_GROW(stack->items, stack->nr + 1, stack->alloc);
+	stack->items[stack->nr++] = commit;
+}
+
+static struct commit *commit_stack_pop(struct commit_stack *stack)
+{
+	return stack->nr ? stack->items[--stack->nr] : NULL;
+}
+
+static void commit_stack_clear(struct commit_stack *stack)
+{
+	FREE_AND_NULL(stack->items);
+	stack->nr = stack->alloc = 0;
+}
+
 void mark_parents_uninteresting(struct commit *commit)
 {
-	struct commit_list *parents = NULL, *l;
+	struct commit_stack pending = COMMIT_STACK_INIT;
+	struct commit_list *l;
 
 	for (l = commit->parents; l; l = l->next)
-		commit_list_insert(l->item, &parents);
+		commit_stack_push(&pending, l->item);
 
-	while (parents) {
-		struct commit *commit = pop_commit(&parents);
+	while (pending.nr > 0) {
+		struct commit *commit = commit_stack_pop(&pending);
 
-		while (commit) {
 		if (commit->object.flags & UNINTERESTING)
-				break;
-
+			return;
 		commit->object.flags |= UNINTERESTING;
 
 		/*
@@ -115,14 +137,11 @@ void mark_parents_uninteresting(struct commit *commit)
 		 * wasn't uninteresting), in which case we need
 		 * to mark its parents recursively too..
 		 */
-			if (!commit->parents)
-				break;
+		for (l = commit->parents; l; l = l->next)
+			commit_stack_push(&pending, l->item);
+	}
 
-			for (l = commit->parents->next; l; l = l->next)
-				commit_list_insert(l->item, &parents);
-			commit = commit->parents->item;
-		}
-	}
+	commit_stack_clear(&pending);
 }
 
 static void add_pending_object_with_path(struct rev_info *revs,