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/build-support/fetchdarcs/builder.sh

@@ -2,9 +2,9 @@ source $stdenv/setup
 
 tagtext=""
 tagflags=""
-if test -n "$tag"; then
-    tagtext="(tag $tag) "
-    tagflags="--tag=$tag"
+if test -n "$rev"; then
+    tagtext="(tag $rev) "
+    tagflags="--tag=$rev"
 elif test -n "$context"; then
     tagtext="(context) "
     tagflags="--context=$context"

pkgs/build-support/fetchdarcs/default.nix

@@ -1,4 +1,4 @@
-{stdenv, darcs, nix}: {url, tag ? null, context ? null, md5 ? "", sha256 ? ""}:
+{stdenv, darcs, nix}: {url, rev ? null, context ? null, md5 ? "", sha256 ? ""}:
 
 stdenv.mkDerivation {
   name = "fetchdarcs";
@@ -9,5 +9,5 @@ stdenv.mkDerivation {
   outputHashMode = "recursive";
   outputHash = if sha256 == "" then md5 else sha256;
   
-  inherit url tag context;
+  inherit url rev context;
 }