Allow refs to start with a digit
Allow refs to start with a digit

E.g. we want to accept "19.03" as a ref.

rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix