Make (--)refresh complain if it results in an empty patch

[tino]

I relatively frequently have some unstaged changes that I thought I staged. I run stg new --refresh --index --message "great work!" and continue. But I didn't see that the patch was now empty! I've even created empty pull requests this way, being happy to see them pass CI only to found out nothing changed....

I'd say running stg refresh or --refresh should error when it results in not making any changes to the patch, as I don't see a case where this could ever be the intention. If there is I'd be happy with a warning too.