fetchbzr, fetchdarcs, fetchhg: use rev attr

This makes it match the behaviour of fetchgit and fetchsvn, so it's
easier to write scripts that support all of them.

pkgs/applications/editors/vim/default.nix

@@ -7,7 +7,7 @@ stdenv.mkDerivation rec {
 
   src = fetchhg {
     url = "https://vim.googlecode.com/hg/";
-    tag = "v7-4-316";
+    rev = "v7-4-316";
     sha256 = "0scxx33p1ky0wihk04xqpd6rygp1crm0hx446zbjwbsjj6xxn7sx";
   };