path-walk API: avoid adding a root tree more than once (#5195)

When adding tree objects, we are very careful to avoid adding the same
tree object more than once. There was one small gap in that logic,
though: when adding a root tree object. Two refs can easily share the
same root tree object, and we should still not add it more than once.

path-walk.c

@@ -278,9 +278,11 @@ int walk_objects_by_path(struct path_walk_info *info)
 			struct object_array_entry *pending = info->revs->pending.objects + i;
 			struct object *obj = pending->item;
 
-			if (obj->type == OBJ_COMMIT)
+			if (obj->type == OBJ_COMMIT || obj->flags & SEEN)
 				continue;
 
+			obj->flags |= SEEN;
+
 			while (obj->type == OBJ_TAG) {
 				struct tag *tag = lookup_tag(info->revs->repo,
 							     &obj->oid);
@@ -341,8 +343,12 @@ int walk_objects_by_path(struct path_walk_info *info)
 		t = lookup_tree(info->revs->repo, oid);
 
 		if (t) {
-			oidset_insert(&root_tree_set, oid);
-			oid_array_append(&root_tree_list->oids, oid);
+			if (t->object.flags & SEEN)
+				continue;
+			t->object.flags |= SEEN;
+
+			if (!oidset_insert(&root_tree_set, oid))
+				oid_array_append(&root_tree_list->oids, oid);
 		} else {
 			warning("could not find tree %s", oid_to_hex(oid));
 		}

t/t6601-path-walk.sh

@@ -276,4 +276,26 @@ test_expect_success 'topic, not base, boundary with pruning' '
 	test_cmp expect.sorted out.sorted
 '
 
+test_expect_success 'trees are reported exactly once' '
+	test_when_finished "rm -rf unique-trees" &&
+	test_create_repo unique-trees &&
+	(
+		cd unique-trees &&
+		mkdir initial &&
+		test_commit initial/file &&
+
+		git switch -c move-to-top &&
+		git mv initial/file.t ./ &&
+		test_tick &&
+		git commit -m moved &&
+
+		git update-ref refs/heads/other HEAD
+	) &&
+
+	test-tool -C unique-trees path-walk -- --all >out &&
+	tree=$(git -C unique-trees rev-parse HEAD:) &&
+	grep "$tree" out >out-filtered &&
+	test_line_count = 1 out-filtered
+'
+
 test_done