tmux: fix truecolor in tmux

So story time, I discovered :checkhealth in Neovim, and it gave a fairly
decent explanation of how to fix terminal truecolor support.

Namely, use `tmux-256color` (or `screen-256color`) for the
default-terminal, then force truecolor (Tc) in the terminal-overrides

It also mentioned that `focus-events` needs to be enabled for n/vim to
detect changes to its buffers. Notably, this allows vim to rerender
changed content when switching into a tmux pane containing a vim
session.

tmux/tmux.conf

@@ -27,10 +27,14 @@ set -g status-right '#T #[fg=yellow,bold]#(uptime -p)#[default] #[fg=cyan,bold]%
 set -g set-titles on
 setw -g aggressive-resize on
 setw -g monitor-activity on
-# The following three makes italics work in tmux
+
 set -g xterm-keys on
-set -g default-terminal "xterm-256color"
-set -as terminal-overrides ',xterm*:sitm=\E[3m'
+# Correctly set up terminal for truecolor support
+set -g default-terminal "tmux-256color"
+set -ga terminal-overrides ',xterm-256color:Tc'
+
+# Fix :h 'autoread' in Vim
+set -g focus-events on
 
 # Refresh status line every 5 seconds
 set -g status-interval 5