When a review request has commit history, it needs to always be updated via RBTools. Attempting to use "Update Diff" to upload a diff file will break in a confusing and useless way. This change makes it so we show an informative error instead of letting the user choose a diff file and then showing them an unknown error. This also gets rid of an obsolete implementation for this that hadn't been removed from ReviewRequestEditorView. Testing Done: Clicked "Update Diff" on a review request with commit history and saw the new error dialog. Did it on a review request without commit history and saw the upload dialog as before.