bash-completion: 2.7 -> 2.8

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.8 with grep in /nix/store/svgfjl0v59ff90l6h2gzbsplhbk3aqhp-bash-completion-2.8
- directory tree listing: https://gist.github.com/23a9bb8eddc5667ef5a94d69ea7ab340