Support for `fetchLFS=true` in `fetchgit`

[Laurent2916]

Hi, since fetchFromGitea doesn't seem to be supported, using fetchgit does the job.
Though if the repository uses LFS the returned hash isn't correct.

For example, the following wouldn't be correctly updated by update-nix-fetchgit:

  pages = pkgs.fetchgit {
    url = "https://gitea.mydomain.com/owner/repo";
    fetchLFS = true;
    rev = "_"; # some_branch
    sha256 = "_";  <-- will correspond to the sha256 of the uninitialized LFS repository
  };