add: allow operating on a sparse-only index

Disable command_requires_full_index for 'git add'. This does not require any additional removals of ensure_full_index(). The main reason is that 'git add' discovers changes based on the pathspec and the worktree itself. These are then inserted into the index directly, and calls to index_name_pos() or index_file_exists() already call expand_to_path() at the appropriate time to support a sparse-index. Add a test to check that 'git add -A' and 'git add <file>' does not expand the index at all, as long as <file> is not within a sparse directory. This does not help the global 'git add .' case. We can measure the improvement using p2000-sparse-operations.sh with these results: Test HEAD~1 HEAD ------------------------------------------------------------------------------ 2000.6: git add -A (full-index-v3) 0.35(0.30+0.05) 0.37(0.29+0.06) +5.7% 2000.7: git add -A (full-index-v4) 0.31(0.26+0.06) 0.33(0.27+0.06) +6.5% 2000.8: git add -A (sparse-index-v3) 0.57(0.53+0.07) 0.05(0.04+0.08) -91.2% 2000.9: git add -A (sparse-index-v4) 0.58(0.55+0.06) 0.05(0.05+0.06) -91.4% While the 91% improvement seems impressive, it's important to recognize that previously we had significant overhead for expanding the sparse-index. Comparing to the full index case, 'git add -A' goes from 0.37s to 0.05s, which is "only" an 86% improvement. This modification to 'git add' creates some behavior change depending on the use of a sparse index. We modify a test in t1092 to demonstrate these changes which will be remedied in future changes. Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>