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.

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.