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/tools/security/meo/default.nix

@@ -5,7 +5,7 @@ stdenv.mkDerivation {
   
   src = fetchhg {
     url = http://oss.stamfest.net/hg/meo;
-    tag = "b48e5f16cff8";
+    rev = "b48e5f16cff8";
     sha256 = "0ifg7y28s89i9gwda6fyj1jbrykbcvq8bf1m6rxmdcv5afi3arbq";
   };