timewarrior: install bash completion

The completions files are not installed by default, so we need to
install them manually.  This is done by adding the installShellFiles
build input and calling installShellCompletion in postInstall.

pkgs/applications/misc/timewarrior/default.nix

@@ -1,4 +1,4 @@
-{ lib, stdenv, fetchFromGitHub, cmake, asciidoctor }:
+{ lib, stdenv, fetchFromGitHub, cmake, asciidoctor, installShellFiles }:
 
 stdenv.mkDerivation rec {
   pname = "timewarrior";
@@ -12,10 +12,15 @@ stdenv.mkDerivation rec {
     fetchSubmodules = true;
   };
 
-  nativeBuildInputs = [ cmake asciidoctor ];
+  nativeBuildInputs = [ cmake asciidoctor installShellFiles ];
 
   dontUseCmakeBuildDir = true;
 
+  postInstall = ''
+    installShellCompletion --cmd timew \
+      --bash completion/timew-completion.bash
+  '';
+
   meta = with lib; {
     description = "A command-line time tracker";
     homepage = "https://timewarrior.net";