sparse-checkout: enable `set` to initialize sparse-checkout mode

sparse-checkout: enable `set` to initialize sparse-checkout mode

The previously suggested workflow:
  git sparse-checkout init ...
  git sparse-checkout set ...

Suffered from three problems:
  1) It would delete nearly all files in the first step, then
     restore them in the second.  That was poor performance and
     forced unnecessary rebuilds.
  2) The two-step process resulted in two progress bars, which
     was suboptimal from a UI point of view for wrappers that
     invoked both of these commands but only exposed a single
     command to their end users.
  3) With cone mode, the first step would delete nearly all
     ignored files everywhere, because everything was considered
     to be outside of the specified sparsity paths.  (The user was
     not allowed to specify any sparsity paths in the `init` step.)

Avoid these problems by teaching `set` to understand the extra
parameters that `init` takes and performing any necessary initialization
if not already in a sparse checkout.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Victoria Dye <vdye@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>