Fix the "Show X Above/Below" when lines contain tabs
Fix the "Show X Above/Below" when lines contain tabs

Expat doesn't recognize the entity we use for horizontal
tabs so we need to escape it and then unescape it before
sending the HTML as a JSON entry to the browser.

Signed-off-by: Shawn O. Pearce <f3ea74d906fa9fe97c1fef6bad9cb871485c7045@google.com>

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