diff-tree: integrate with sparse index

The index is read in 'cmd_diff_tree' at two points:

1. The first index read was added in fd66bcc (diff-tree: read the
index so attribute checks work in bare repositories, 2017-12-06) to deal
with reading '.gitattributes' content. 77efbb3 (attr: be careful
about sparse directories, 2021-09-08) established that, in a sparse
index, we do _not_ try to load a '.gitattributes' file from within a
sparse directory.

2. The second index access point is involved in rename detection,
specifically when reading from stdin.This was initially added in
f0c6b2a ([PATCH] Optimize diff-tree -[CM]--stdin, 2005-05-27), where
'setup' was set to 'DIFF_SETUP_USE_SIZE_CACHE |DIFF_SETUP_USE_CACHE'.
That assignment was later modified to drop the'DIFF_SETUP_USE_CACHE' in
ff7fe37 (diff.c: move read_index() code back to the caller,
2018-08-13).However, 'DIFF_SETUP_USE_SIZE_CACHE' seems to be unused as
of 6e0b8ed (diff.c: do not use a separate "size cache"., 2007-05-07)
and nothing about 'detect_rename' otherwise indicates index usage.

Hence we can just set the requires-full-index to false for "diff-tree".

Add tests that verify that 'git diff-tree' behaves correctly when the
sparse index is enabled and test to ensure the index is not expanded.

The `p2000` tests demonstrate a ~98% execution time reduction for
'git diff-tree' using a sparse index:

Test                                                before  after
-----------------------------------------------------------------------
2000.94: git diff-tree HEAD (full-v3)                0.05   0.04 -20.0%
2000.95: git diff-tree HEAD (full-v4)                0.06   0.05 -16.7%
2000.96: git diff-tree HEAD (sparse-v3)              0.59   0.01 -98.3%
2000.97: git diff-tree HEAD (sparse-v4)              0.61   0.01 -98.4%
2000.98: git diff-tree HEAD -- f2/f4/a (full-v3)     0.05   0.05 +0.0%
2000.99: git diff-tree HEAD -- f2/f4/a (full-v4)     0.05   0.04 -20.0%
2000.100: git diff-tree HEAD -- f2/f4/a (sparse-v3)  0.58   0.01 -98.3%
2000.101: git diff-tree HEAD -- f2/f4/a (sparse-v4)  0.55   0.01 -98.2%

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

Author: Cheskaqiqi

builtin/diff-tree.c

@@ -122,6 +122,10 @@ int cmd_diff_tree(int argc, const char **argv, const char *prefix)
 		usage(diff_tree_usage);
 
 	git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
+
+	prepare_repo_settings(the_repository);
+	the_repository->settings.command_requires_full_index = 0;
+
 	repo_init_revisions(the_repository, opt, prefix);
 	if (repo_read_index(the_repository) < 0)
 		die(_("index file corrupt"));

t/perf/p2000-sparse-operations.sh

@@ -131,5 +131,7 @@ 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 diff-tree HEAD
+test_perf_on_all git diff-tree HEAD -- $SPARSE_CONE/a
 
 test_done

t/t1092-sparse-checkout-compatibility.sh

@@ -2170,4 +2170,46 @@ test_expect_success 'sparse index is not expanded: diff-files' '
 	ensure_not_expanded diff-files -- "deep/*"
 '
 
+test_expect_success 'diff-tree' '
+	init_repos &&
+
+	# Test change inside sparse cone
+	tree1=$(git -C sparse-index rev-parse HEAD^{tree}) &&
+	tree2=$(git -C sparse-index rev-parse update-deep^{tree}) &&
+	test_all_match git diff-tree $tree1 $tree2 &&
+	test_all_match git diff-tree $tree1 $tree2 -- deep/a &&
+	test_all_match git diff-tree HEAD update-deep &&
+	test_all_match git diff-tree HEAD update-deep -- deep/a &&
+
+	# Test change outside sparse cone
+	tree3=$(git -C sparse-index rev-parse update-folder1^{tree}) &&
+	test_all_match git diff-tree $tree1 $tree3 &&
+	test_all_match git diff-tree $tree1 $tree3 -- folder1/a &&
+	test_all_match git diff-tree HEAD update-folder1 &&
+	test_all_match git diff-tree HEAD update-folder1 -- folder1/a &&
+
+	# Check that SKIP_WORKTREE files are not materialized
+	test_path_is_missing sparse-checkout/folder1/a &&
+	test_path_is_missing sparse-index/folder1/a &&
+	test_path_is_missing sparse-checkout/folder2/a &&
+	test_path_is_missing sparse-index/folder2/a
+'
+
+test_expect_success 'sparse-index is not expanded: diff-tree' '
+	init_repos &&
+
+	tree1=$(git -C sparse-index rev-parse HEAD^{tree}) &&
+	tree2=$(git -C sparse-index rev-parse update-deep^{tree}) &&
+	tree3=$(git -C sparse-index rev-parse update-folder1^{tree}) &&
+
+	ensure_not_expanded diff-tree $tree1 $tree2 &&
+	ensure_not_expanded diff-tree $tree1 $tree2 -- deep/a &&
+	ensure_not_expanded diff-tree HEAD update-deep &&
+	ensure_not_expanded diff-tree HEAD update-deep -- deep/a &&
+	ensure_not_expanded diff-tree $tree1 $tree3 &&
+	ensure_not_expanded diff-tree $tree1 $tree3 -- folder1/a &&
+	ensure_not_expanded diff-tree HEAD update-folder1 &&
+	ensure_not_expanded diff-tree HEAD update-folder1 -- folder1/a
+'
+
 test_done