msg: %-double prefix
msg: %-double prefix

All the existing callers use literal strings without any percent
characters, but we should protect against future callers including
a percent in the prefix string, too.

bradparks/tmux-MacOSX-pasteboard,tjt263/tmux-MacOSX-pasteboard,lembacon/tmux-MacOSX-pasteboard,ChrisJohnsen/tmux-MacOSX-pasteboard