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 <[email protected]>
Signed-off-by: Shuqi Liang <[email protected]>
Acked-by: Victoria Dye <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Cheskaqiqi

builtin/worktree.c

@@ -1200,5 +1200,9 @@ int cmd_worktree(int ac, const char **av, const char *prefix)
 		prefix = "";
 
 	ac = parse_options(ac, av, prefix, options, git_worktree_usage, 0);
+
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	return fn(ac, av, prefix);
 }

t/perf/p2000-sparse-operations.sh

@@ -131,5 +131,6 @@ test_perf_on_all git describe --dirty
 test_perf_on_all 'echo >>new && git describe --dirty'
 test_perf_on_all git diff-files
 test_perf_on_all git diff-files -- $SPARSE_CONE/a
+test_perf_on_all "git worktree add ../temp && git worktree remove ../temp"
 
 test_done

t/t1092-sparse-checkout-compatibility.sh

@@ -2180,4 +2180,41 @@ test_expect_success 'sparse index is not expanded: diff-files' '
 	ensure_not_expanded diff-files -- "deep/*"
 '
 
+test_expect_success 'worktree' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	for repo in full-checkout sparse-checkout sparse-index
+	do
+		worktree=${repo}-wt &&
+		git -C $repo worktree add ../$worktree &&
+
+		# Compare worktree content with "ls"
+		(cd $repo && ls) >worktree_contents &&
+		(cd $worktree && ls) >new_worktree_contents &&
+		test_cmp worktree_contents new_worktree_contents &&
+
+		# Compare index content with "ls-files --sparse"
+		git -C $repo ls-files --sparse >index_contents &&
+		git -C $worktree ls-files --sparse >new_index_contents &&
+		test_cmp index_contents new_index_contents &&
+
+		git -C $repo worktree remove ../$worktree || return 1
+	done &&
+
+	test_all_match git worktree add .worktrees/hotfix &&
+	run_on_all ../edit-contents .worktrees/hotfix/deep/a &&
+	test_all_match test_must_fail git worktree remove .worktrees/hotfix
+'
+
+test_expect_success 'worktree is not expanded' '
+	init_repos &&
+
+	ensure_not_expanded worktree add .worktrees/hotfix &&
+	ensure_not_expanded worktree remove .worktrees/hotfix
+'
+
 test_done