add border-native option to --tmux flag

[aead]

This commit adds the border-native resulting in
the following:

--tmux[=[center|top|bottom|left|right][,SIZE[%]][,SIZE[%]][,border-native]]

By default, when not specified, the -B flag is passed to the
tmux popup-window command such that no border is drawn around
the tmux popup window.

When the border-native option is present the -B flag is omitted
and the popup window is drawn using the border style configured in
the tmux config file.

Fixes #4156

[junegunn]

I updated the branch to prepend --margin 0,1 so that it looks similar to when --border is used. What do you think? fzf-tmux used to do that before tmux 3.3 when there was no way to disable the native border.

fzf --tmux
fzf --tmux border-native
fzf --tmux --list-border
fzf --tmux border-native --list-border

One gotcha is that the extra margin is not colored when you use a non-default background color, but I don't think it's a critical problem.

fzf --tmux --color bg:#ff0000

fzf --tmux border-native --color bg:#ff0000

fzf --tmux border-native --color bg:#ff0000 --margin 0

[aead]

The added margin does make it look nicer! For example,

And with set -g popup-border-lines rounded and coloring one can make both borders identical.

One gotcha is that the extra margin is not colored when you use a non-default background color, but I don't think it's a critical problem.

Yeah, and with --margin 0 you can turn it off if you really want to.

Since the native border is styled by tmux, FZF could forward its --border and --color styles to tmux to adjust things dynamically with tmux popup -S -b. However, even if, this may require a separate PR.

[junegunn]

Merged, thanks!