Fix tmux is_vim variable for nested vim sessions
Fix tmux is_vim variable for nested vim sessions

Previously if a vim session was launched by a separate command like
`brew edit`, it wasn't working with vim-tmux-navigator. I'm not sure why
the escaping that does work is different from the upstream readme.

keith/dotfiles,keith/dotfiles,keith/dotfiles,keith/dotfiles,keith/dotfiles,keith/dotfiles