resolve revision when using submodules

[teto]

I have started using nurl in luarocks-nix.

$ nurl https://github.com/teto/luabitop master -S

fetchFromGitHub {
  owner = "teto";
  repo = "luabitop";
  rev = "master";
  hash = "sha256-yLE198w1Ry4y9kflK/QHeWlmi0T7zJSie00UVgR/l54=";
  fetchSubmodules = true;
}

ofc it doesn't build

       last 13 log lines:
       > exporting https://github.com/teto/luabitop.git (rev master) into /nix/store/20zhsmkxm7nndlprrqbchknghjlz4cv1-source
       > Initialized empty Git repository in /nix/store/20zhsmkxm7nndlprrqbchknghjlz4cv1-source/.git/
       > fatal: couldn't find remote ref refs/tags/master
       > remote: Enumerating objects: 33, done.
       > remote: Counting objects: 100% (33/33), done.
       > remote: Compressing objects: 100% (29/29), done.
       > remote: Total 33 (delta 2), reused 22 (delta 2), pack-reused 0
       > Unpacking objects: 100% (33/33), 37.73 KiB | 402.00 KiB/s, done.
       > From https://github.com/teto/luabitop
       >  * branch            HEAD       -> FETCH_HEAD
       > fatal: Not a valid object name
       > Unrecognized git object type:
       > Unable to checkout refs/tags/master from https://github.com/teto/luabitop.git.
       For full logs, run 'nix log /nix/store/lmr3pxlqnd82169xlr7sdsb5ddq5nbvl-source.drv'.

would be nice if nurl could infer the revision. Related to #5