worktree: integrate with sparse-index

The index is read in 'worktree.c' at two points: 1.The 'validate_no_submodules' function, which checks if there are any submodules present in the worktree. 2.The 'check_clean_worktree' function, which verifies if a worktree is 'clean', i.e., there are no untracked or modified but uncommitted files. This is done by running the 'git status' command, and an error message is thrown if the worktree is not clean. Given that 'git status' is already sparse-aware, the function is also sparse-aware. Hence we can just set the requires-full-index to false for "git worktree". Add tests that verify that 'git worktree' behaves correctly when the sparse index is enabled and test to ensure the index is not expanded. The `p2000` tests demonstrate a ~20% execution time reduction for 'git worktree' using a sparse index: (Note:the p2000 test results didn't reflect the huge speedup because of the index reading time is minuscule comparing to the filesystem operations.) Test before after ----------------------------------------------------------------------- 2000.102: git worktree add....(full-v3) 3.15 2.82 -10.5% 2000.103: git worktree add....(full-v4) 3.14 2.84 -9.6% 2000.104: git worktree add....(sparse-v3) 2.59 2.14 -16.4% 2000.105: git worktree add....(sparse-v4) 2.10 1.57 -25.2% Helped-by: Victoria Dye <vdye@github.com> Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com> Acked-by: Victoria Dye <vdye@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>