checkout: stop expanding sparse indexes

Previous changes did the necessary improvements to unpack-trees.c and diff-lib.c in order to modify a sparse index based on its comparision with a tree. The only remaining work is to remove some ensure_full_index() calls and add tests that verify that the index is not expanded in our interesting cases. Include 'switch' and 'restore' in these tests, as they share a base implementation with 'checkout'. Here are the relevant performance results from p2000-sparse-operations.sh: Test HEAD~1 HEAD -------------------------------------------------------------------------------- 2000.18: git checkout -f - (full-v3) 0.49(0.43+0.03) 0.47(0.39+0.05) -4.1% 2000.19: git checkout -f - (full-v4) 0.45(0.37+0.06) 0.42(0.37+0.05) -6.7% 2000.20: git checkout -f - (sparse-v3) 0.76(0.71+0.07) 0.04(0.03+0.04) -94.7% 2000.21: git checkout -f - (sparse-v4) 0.75(0.72+0.04) 0.05(0.06+0.04) -93.3% It is important to compare the full index case to the sparse index case, as the previous results for the sparse index were inflated by the index expansion. For index v4, this is an 88% improvement. On an internal repository with over two million paths at HEAD and a sparse-checkout definition containing ~60,000 of those paths, 'git checkout' went from 3.5s to 297ms with this change. The theoretical optimum where only those ~60,000 paths exist was 275ms, so the extra sparse directory entries contribute a 22ms overhead. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>