mutagen: add shell completions

pkgs/tools/misc/mutagen/default.nix

@@ -1,4 +1,4 @@
-{ lib, buildGoModule, fetchFromGitHub, fetchzip }:
+{ lib, buildGoModule, fetchFromGitHub, fetchzip, installShellFiles }:
 
 buildGoModule rec {
   pname = "mutagen";
@@ -24,6 +24,8 @@ buildGoModule rec {
     hash = "sha256-RFB1/gzLjs9w8mebEd4M9Ldv3BrLIj2RsN/QAIJi45E=";
   };
 
+  nativeBuildInputs = [ installShellFiles ];
+
   doCheck = false;
 
   subPackages = [ "cmd/mutagen" "cmd/mutagen-agent" ];
@@ -33,6 +35,17 @@ buildGoModule rec {
   postInstall = ''
     install -d $out/libexec
     ln -s ${agents}/mutagen-agents.tar.gz $out/libexec/
+
+    $out/bin/mutagen generate \
+      --bash-completion-script mutagen.bash \
+      --fish-completion-script mutagen.fish \
+      --zsh-completion-script mutagen.zsh
+
+    installShellCompletion \
+      --cmd mutagen \
+      --bash mutagen.bash \
+      --fish mutagen.fish \
+      --zsh mutagen.zsh
   '';
 
   meta = with lib; {