tig: enable bash completion

Copy the bash completion script that comes with tig to
$out/etc/bash_completion.d/.

Note: to enable bash completion in NixOS, set
environment.enableBashCompletion = true;

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

@@ -10,6 +10,8 @@ stdenv.mkDerivation rec {
   installPhase = ''
     make install
     make install-doc
+    mkdir -p $out/etc/bash_completion.d/
+    cp contrib/tig-completion.bash $out/etc/bash_completion.d/
   '';
   meta = {
     homepage = "http://jonas.nitro.dk/tig/";