tig: Don't install the broken ZSH completion

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: https://github.com/jonas/tig/issues/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).

pkgs/applications/version-management/git-and-tools/tig/default.nix

@@ -34,13 +34,14 @@ stdenv.mkDerivation rec {
     # fixes tig-completion __git-complete dependency
     sed -i '1s;^;source ${git}/share/bash-completion/completions/git\n;' contrib/tig-completion.bash
 
-    substituteInPlace contrib/tig-completion.zsh \
-      --replace 'e=$(dirname ''${funcsourcetrace[1]%:*})/tig-completion.bash' "e=$out/share/bash-completion/completions/tig"
-
     install -D contrib/tig-completion.bash $out/share/bash-completion/completions/tig
-    install -D contrib/tig-completion.zsh $out/share/zsh/site-functions/_tig
     cp contrib/vim.tigrc $out/etc/
 
+    # Note: Until https://github.com/jonas/tig/issues/940 is resolved it is best
+    # not to install the ZSH completion so that the fallback implemenation from
+    # ZSH can be used (Completion/Unix/Command/_git: "_tig () { _git-log }"):
+    #install -D contrib/tig-completion.zsh $out/share/zsh/site-functions/_tig
+
     wrapProgram $out/bin/tig \
       --prefix PATH ':' "${git}/bin"
   '';