fetchhg: add option to fetch hg subrepos

pkgs/build-support/fetchhg/default.nix

@@ -1,4 +1,4 @@
-{stdenv, mercurial, nix}: {name ? null, url, rev ? null, md5 ? null, sha256 ? null}:
+{stdenv, mercurial, nix}: {name ? null, url, rev ? null, md5 ? null, sha256 ? null, fetchSubrepos ? false}:
 
 # TODO: statically check if mercurial as the https support if the url starts woth https.
 stdenv.mkDerivation {
@@ -9,10 +9,12 @@ stdenv.mkDerivation {
   # Nix <= 0.7 compatibility.
   id = md5;
 
+  subrepoClause = if fetchSubrepos then "S" else "";
+
   outputHashAlgo = if md5 != null then "md5" else "sha256";
   outputHashMode = "recursive";
   outputHash = if md5 != null then md5 else sha256;
-  
+
   inherit url rev;
   preferLocalBuild = true;
 }