Fix hash mismatch when the repo of a dependency has submodules

[ento]

The mismatch happened when I tried to build segmentio/chamber, which depends on segmentio/backo-go.

The error looks like:

fixed-output derivation produced path '/nix/store/...' with sha256 hash '\' instead of the expected hash '\'

How this happened, to my understanding:

• buildGoPackage calls fetchgit with only url, rev, and sha256 specified: https://github.com/NixOS/nixpkgs/blob/8d8e56824de52a0c7a64d2ad2c4ed75ed85f446a/pkgs/development/go-modules/generic/default.nix#L54-L56
• fetchgit's default for fetchSubmodules is true: https://github.com/NixOS/nixpkgs/blob/8d8e56824de52a0c7a64d2ad2c4ed75ed85f446a/pkgs/build-support/fetchgit/default.nix#L16
• while nix-prefetch-git script's default is false: https://github.com/NixOS/nixpkgs/blob/8d8e56824de52a0c7a64d2ad2c4ed75ed85f446a/pkgs/build-support/fetchgit/nix-prefetch-git#L11

Until the mismatch in defaults is fixed upstream, which is tracked here, the easiest fix is to match how we call nix-prefetch-git with how fetchgit is called.

[adisbladis]

LGTM! Thank you!