[Search Git Status] use box drawing chars for staged/unstaged header

[yogan]

This makes the little header box for Git staged/unstaged/merged/etc. look a bit nicer.

Old:

+--------+
| Staged |
+--------+

New:

╭────────╮
│ Staged │
╰────────╯

It does not look perfect on GitHub web, but works in a terminal with a proper font:

We could make this optional (add some $_fzf_fish_use_unicode_box variable), but fzf itself is
also using those box drawing characters by default, so I guess pretty much everyone using fzf.fish
should be fine with it.

[PatrickF1]

Hi Frank, thanks for the PR. This is a great idea!

1. How do you know fzf also uses these box chars already? Are they the ones used in the fzf window border?
2. How do you type the box characters on the keyboard?

[PatrickF1]

Hmm I forgot that it seems we are already using these chars here https://github.com/PatrickF1/fzf.fish/blob/main/functions/_fzf_search_history.fish#L11

[yogan]

Hi Patrick! Yes, I have used the same characters that fzf does, and that's also how I "typed" them: by copy/paste from a fzf output. :-)
There are ways to type extended Unicode characters by some key combinations, but that depends on the OS, so I can't really give a clear answer for that. I usually just look them up when I need them. Wikipedia has a nice overview for example: https://en.wikipedia.org/wiki/Box-drawing_character

Those are the ones that I have used:

$ echo '╭─╮│╰─╯' | unicode-inspect.pl
╭       U256d # BOX DRAWINGS LIGHT ARC DOWN AND RIGHT
─       U2500 # BOX DRAWINGS LIGHT HORIZONTAL
╮       U256e # BOX DRAWINGS LIGHT ARC DOWN AND LEFT
│       U2502 # BOX DRAWINGS LIGHT VERTICAL
╰       U2570 # BOX DRAWINGS LIGHT ARC UP AND RIGHT
─       U2500 # BOX DRAWINGS LIGHT HORIZONTAL
╯       U256f # BOX DRAWINGS LIGHT ARC UP AND LEFT