sparse-checkout: integrate with sparse index

When modifying the sparse-checkout definition, the sparse-checkout builtin calls update_sparsity() to modify the SKIP_WORKTREE bits of all cache entries in the index. Before, we needed the index to be fully expanded in order to ensure we had the full list of files necessary that match the new patterns. Insert a call to reset_sparse_directories() that expands sparse directories that are within the new pattern list, but only far enough that every necessary file path now exists as a cache entry. The remaining logic within update_sparsity() will modify the SKIP_WORKTREE bits appropriately. This allows us to disable command_requires_full_index within the sparse-checkout builtin. Add tests that demonstrate that we are not expanding to a full index unnecessarily. We can see the improved performance in the p2000 test script: Test HEAD~1 HEAD ------------------------------------------------------------------------ 2000.24: git ... (sparse-v3) 2.14(1.55+0.58) 1.57(1.03+0.53) -26.6% 2000.25: git ... (sparse-v4) 2.20(1.62+0.57) 1.58(0.98+0.59) -28.2% These reductions of 26-28% are small compared to most examples, but the time is dominated by writing a new copy of the base repository to the worktree and then deleting it again. The fact that the previous index expansion was such a large portion of the time is telling how important it is to complete this sparse index integration. Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>