Merge branch 'ds/sparse-checkout-requires-per-worktree-config'

Merge branch 'ds/sparse-checkout-requires-per-worktree-config'

"git sparse-checkout" wants to work with per-worktree configuration,
but did not work well in a worktree attached to a bare repository.

* ds/sparse-checkout-requires-per-worktree-config:
  config: make git_configset_get_string_tmp() private
  worktree: copy sparse-checkout patterns and config on add
  sparse-checkout: set worktree-config correctly
  config: add repo_config_set_worktree_gently()
  worktree: create init_worktree_config()
  Documentation: add extensions.worktreeConfig details