fzf: update to 0.28.0.

0.28.0
------
- Added `--header-first` option to print header before the prompt line
  ```sh
  fzf --header $'Welcome to fzf\n������������������������������������������' --reverse --height 30% --border --header-first
  ```
- Added `--scroll-off=LINES` option (similar to `scrolloff` option of Vim)
    - You can set it to a very large number so that the cursor stays in the
      middle of the screen while scrolling
      ```sh
      fzf --scroll-off=5
      fzf --scroll-off=999
      ```
- Fixed bug where preview window is not updated on `reload` (#2644)
- fzf on Windows will also use `$SHELL` to execute external programs
    - See #2638 and #2647
    - Thanks to @rashil2000, @vovcacik, and @janlazo

(wiz)