There are some Nixpkgs specific issues that we could fix but due to
changes in Git it currently isn't possible to source git-completion.bash
from ZSH (at least not how tig-completion.bash expects it).
Upstream issue: jonas/tig#940

For the meantime it seems best to simply not install it anymore so that
the fallback implementation from ZSH can be used (more inaccurate as the
git-log completion is reused but it is helpful to complete remotes,
branches, tags, etc. and doesn't emit an error to the console).

Motivation for this change

Without this change (errors and broken output when trying to use the completion):

$ tig orWARNING: this script is deprecated, please see git-completion.zsh
__git_complete: command not found

$ tig __tig_main: command not found
                tig ori__tig_main: command not found
file            tig ori__tig_main: command not found
Sorry, no matchetig ori   

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

cc @bjornfor @domenkozar @qknight @globin