Import upstream commit to make emacs-in-tmux-in-tmux show cursor.

Import upstream commit 2c53b23d59 to make the cursor appear when running
Emacs in a tmux nested inside another tmux session (this is particularly
useful when trying to edit a text file on a remote server that is also
running tmux).

Approved by: mat (maintainer)
Differential Revision: https://reviews.freebsd.org/D3408