Make fetchhg do the same as nix-prefetch-hg, use hg archive. Fixes #2781.

pkgs/build-support/fetchhg/builder.sh

@@ -2,8 +2,9 @@ source $stdenv/setup
 
 header "getting $url${tag:+ ($tag)} into $out"
 
-hg clone --insecure ${tag:+-r "$tag"} "$url" "$out"
+hg clone --insecure ${tag:+-r "$tag"} "$url" hg-clone
 
-rm -rf "$out/.hg"
+hg archive -q -y ${tag:+-r "$tag"} --cwd hg-clone $out
+rm -f $out/.hg_archival.txt
 
 stopNest