message: [vcpkg] Allow shallow git registries
  'git fetch' normally refuses to deal with shallow repositories.
  For vcpkg, this makes it impossible to use shallow clones of remote
  registries such as 'VCPKG_INSTALLATION_ROOT' on Azure Pipelines.
  An extra parameter makes 'git fetch' accept shallow repositories.
diff: @@ -847,8 +847,11 @@ If you wish to silence this error and use classic mode, you can:
  std::error_code ec;
  Files::ExclusiveFileLock guard(Files::ExclusiveFileLock::Wait::Yes, fs, lock_file, ec);
  - System::Command fetch_git_ref =
  - git_cmd_builder(*this, dot_git_dir, work_tree).string_arg("fetch").string_arg("--").string_arg(repo);
  + System::Command fetch_git_ref = git_cmd_builder(*this, dot_git_dir, work_tree)
  + .string_arg("fetch")
  + .string_arg("--update-shallow")
  + .string_arg("--")
  + .string_arg(repo);
  if (treeish.size() != 0)
  {
  fetch_git_ref.string_arg(treeish);
  