fish: add missing deps

Start fish and run "ls". It will complain about not being able to run
which, nroff and gettext. This fixes it.

pkgs/shells/fish/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, autoconf, ncurses, which }:
+{ stdenv, fetchurl, autoconf, ncurses, which, groff, gettext }:
 
 stdenv.mkDerivation rec {
   name = "fish-2.0.0";
@@ -16,6 +16,14 @@ stdenv.mkDerivation rec {
     autoconf
   '';
 
+  postInstall = ''
+    sed -i "s|which |command -v |" "$out/share/fish/functions/type.fish"
+    sed -i "s|nroff |${groff}/bin/nroff |" "$out/share/fish/functions/__fish_print_help.fish"
+    sed -e "s|gettext |${gettext}/bin/gettext |" \
+        -e "s|which |command -v |" \
+        -i "$out/share/fish/functions/_.fish"
+  '';
+
   meta = with stdenv.lib; {
     description = "Smart and user-friendly command line shell";
     homepage = http://fishshell.com/;