Autosave continues to trigger for published posts when no changes applied

[gziolo]

Describe the bug

I discovered that autosave triggers even when there were no changes applied only for the published post. It works perfectly fine for drafts.

To Reproduce
Steps to reproduce the behavior:

1. Create a post (it isn't related to demo post only), write some content and publish it.
2. Reopen your published post.
3. Open network tab in your browser's developer tools.
4. Change something in one of the blocks.
5. Observer autosave being triggered after your changes were autosaved.

Expected behavior
Autosave is triggered only once after content in the published post has changed.

Screenshots

Desktop (please complete the following information):

• OS: macOS Mojave
• Browser: Chrome
• Version: 70

[aduth]

It's technically expected that the post remains "dirty" after an autosave occurs, because it is not a full save for a published post. So while we expect the user should be prompted about unsaved changes (which occurs in the classic editor as well), the autosaves themselves should discontinue after the first.

[aduth]

This may require some new state to distinguish a post which is dirty but for which a user has not made changes since the last save, to be used in considering whether the AutosaveMonitor component should schedule an autosave. In other words, a post may be dirty (prompt on navigate) but not a candidate for autosave.

Reducer pseudo-code:

function hasUnsavedUserEdits( state = false, action ) {
	switch ( action.type ) {
		case 'REQUEST_POST_UPDATE_SUCCESS':
			return false;

		case 'EDIT_POST':
		/* ... all other user edit actions as `case` */
			return true;
	}

	return state;
}

There's some conceptual overlap with the getReferenceByDistinctEdits selector, and it's not necessarily safe to assume that some actions are in-fact user-initiated, so there might be some alternative I'm overlooking here.

[aduth]

Pull request at #12624