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/system/storebrowse/default.nix

@@ -14,7 +14,7 @@ stdenv.mkDerivation rec {
   # This source has license BSD
   srcGoSqlite = fetchhg {
     url = "https://code.google.com/p/gosqlite/";
-    tag = "5baefb109e18";
+    rev = "5baefb109e18";
     sha256 = "0mqfnx06jj15cs8pq9msny2z18x99hgk6mchnaxpg343nzdiz4zk";
   };