fsck: drop unused parameter from traverse_one_object()

Also add comments to seemingly unsafe pointer dereferences, that
are all safe.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fsck.c

@@ -74,7 +74,13 @@ static int mark_object(struct object *obj, int type, void *data)
 {
 	struct object *parent = data;
 
+	/*
+	 * The only case data is NULL or type is OBJ_ANY is when
+	 * mark_object_reachable() calls us.  All the callers of
+	 * that function has non-NULL obj hence ...
+	 */
 	if (!obj) {
+		/* ... these references to parent->fld are safe here */
 		printf("broken link from %7s %s\n",
 			   typename(parent->type), sha1_to_hex(parent->sha1));
 		printf("broken link from %7s %s\n",
@@ -84,6 +90,7 @@ static int mark_object(struct object *obj, int type, void *data)
 	}
 
 	if (type != OBJ_ANY && obj->type != type)
+		/* ... and the reference to parent is safe here */
 		objerror(parent, "wrong object type in link");
 
 	if (obj->flags & REACHABLE)
@@ -109,7 +116,7 @@ static void mark_object_reachable(struct object *obj)
 	mark_object(obj, OBJ_ANY, NULL);
 }
 
-static int traverse_one_object(struct object *obj, struct object *parent)
+static int traverse_one_object(struct object *obj)
 {
 	int result;
 	struct tree *tree = NULL;
@@ -138,7 +145,7 @@ static int traverse_reachable(void)
 		entry = pending.objects + --pending.nr;
 		obj = entry->item;
 		parent = (struct object *) entry->name;
-		result |= traverse_one_object(obj, parent);
+		result |= traverse_one_object(obj);
 	}
 	return !!result;
 }
@@ -556,8 +563,8 @@ static int fsck_cache_tree(struct cache_tree *it)
 			      sha1_to_hex(it->sha1));
 			return 1;
 		}
-		mark_object_reachable(obj);
 		obj->used = 1;
+		mark_object_reachable(obj);
 		if (obj->type != OBJ_TREE)
 			err |= objerror(obj, "non-tree in cache-tree");
 	}