Why not `tmux-256color`?

Now it is
```sh
# Upgrade $TERM
set -g default-terminal "screen-256color"
```
but why not
```sh
# Upgrade $TERM
set -g default-terminal "tmux-256color"
```
tmux color term enables italic font and ensure more color consistency with `xterm-256color` which is usually the case outside `tmux`.