[tmux] Fix true color support, especially in nvim

Tmux needs Tc set to enable truecolor support. At the same time, the
file was setting the same variable multiple times and in illogical ways.
So it now gets the term env var from the actual variable and simply adds
truecolor support.

Also removed cursor switching support for nvim, since tmux seems to
support it natively now. If it does not, this needs to be reinstituted.

Following discussions here:
https://www.reddit.com/r/neovim/comments/825dj7/the_endless_litany_of_tmux_and_nvim_color_problems/
and here:
https://stackoverflow.com/questions/41783367/tmux-tmux-true-color-is-not-working-properly

tmux/.config/tmux/tmux.conf

@@ -21,12 +21,16 @@ setw -g pane-base-index 1
 set -g renumber-windows on
 
 # Terminal improvements
-set -g terminal-overrides "xterm*:XT:smcup@:rmcup@"
 set-window-option -g automatic-rename on
 set-option -g set-titles on
 set -g mouse on
-# set -g default-terminal xterm-256color
-set -g default-terminal tmux-256color
+set -s default-terminal $TERM
+# allow truecolor support
+set -as terminal-overrides ",*:Tc"
+# Set Cursor terminal override, so nvim can change cursor style when changing modes
+# more info see nvim, :help tui-cursor-shape
+# LEGACY: this should not be necessary anymore
+# set -ga terminal-overrides '*:Ss=\E[%p1%d q:Se=\E[ q'
 set -g status-keys vi
 set -g history-limit 10000
 
@@ -38,11 +42,6 @@ setw -g aggressive-resize on
 # No delay for escape key press
 set -sg escape-time 0
 
-# enable true color support for nvim
-set -ga terminal-overrides ',xterm-256color:RGB'
-# Set Cursor terminal override, so nvim can change cursor style when changing modes
-# more info see nvim, :help tui-cursor-shape
-set -ga terminal-overrides '*:Ss=\E[%p1%d q:Se=\E[ q'
 
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