Ref T13161. Ref T12822. Today, we use invisible Zero-Width Spaces to try to improve copy/paste behavior from Differential.
After D20188, we no longer need ZWS characters to avoid copying line numbers. Get rid of these secret invisible semantic ZWS characters completely.
This means that both the left-hand and right-hand side of diffs become copyable, which isn't desired. I'll fix that with a hundred thousand lines of Javascript in the next change: this is a step toward everything working better, but doesn't fix everything yet.