Avoid leaving subdoc defaults on top-level doc when setting subdocument to same value

[vkarpov15]

Summary

The cause of #14722 is that, when creating a new subdocument, Mongoose first applies defaults to the subdocument, and those defaults bubble up to the top-level document. Which leaves any default paths in default state on the top-level document, even if we're setting the subdocument to the exact same value.

This PR makes it so that we skip parent change tracking on defaults when creating a completely new subdocument - the idea is, if we're creating a new subdocument, we'll be setting the subdocument to the top-level document, so we will track the change as "set the subdocument" rather than "set the subdocument, and also set this default on the subdocument."

There's also some followup work I want to do related to #4145: #4145 succeeds because we leave document array underneath a nested path in default state, even though we modify the document array.

Examples

[hasezoey]

I dont fully understand what is fixed here, but the code looks good to me.

[vkarpov15]

Lol. To put it another way, the issue is that, when you do the following, user.$isDefault('updater._id') will be true even though updater was set to the exact same value it was already.

const getUser = () => ({
    _id: new mongoose.Types.ObjectId("66852317541ef0e22ae5214c"),
    name: "blowfishlol",
    email: "[email protected]"
});

const { _id } = await TestModel.create({ name: 'foobar', updater: getUser() });

const user = await TestModel.findById(_id).orFail();
user.set('updater', getUser());

user.$isDefault('updater._id'); // true without this PR, which shouldn't happen since we set `updater` to the same value