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>