glab: install shell completions (#168166)

* glab: install shell completions

* Update pkgs/applications/version-management/git-and-tools/glab/default.nix

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

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

@@ -1,4 +1,4 @@
-{ lib, buildGoModule, fetchFromGitHub }:
+{ lib, buildGoModule, fetchFromGitHub, installShellFiles, stdenv }:
 
 buildGoModule rec {
   pname = "glab";
@@ -24,6 +24,15 @@ buildGoModule rec {
 
   subPackages = [ "cmd/glab" ];
 
+  nativeBuildInputs = [ installShellFiles ];
+
+  postInstall = lib.optionalString (stdenv.hostPlatform == stdenv.buildPlatform) ''
+    installShellCompletion --cmd glab \
+      --bash <($out/bin/glab completion -s bash) \
+      --fish <($out/bin/glab completion -s fish) \
+      --zsh <($out/bin/glab completion -s zsh)
+  '';
+
   meta = with lib; {
     description = "An open-source GitLab command line tool";
     license = licenses.mit;