fix(tmux): escape curlies for tmux v3
fix(tmux): escape curlies for tmux v3

Otherwise, our mouse bindings won't work:

    * INCOMPATIBLE: Add a new {} syntax to the configuration file. This is a string
      similar to single quotes but also includes newlines and allows commands that
      take other commands as string arguments to be expressed more clearly and
      without additional escaping.

      A literal { and } or a string containing { or } must now be escaped or
      quoted, for example '{' and '}' instead of { or }, or 'X#{foo}' instead of
      X#{foo}.

https://github.com/tmux/tmux/blob/master/CHANGES

I've already moved off of 2.9 on this machine, but I believe this should
work on v2 too.

wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent,wincent/wincent