[PATCH v3] write-tree: integrate with sparse index

Update 'git write-tree' to allow using the sparse-index in memory without expanding to a full one.

The recursive algorithm for update_one() was already updated in 2de37c5 (cache-tree: integrate with sparse directory entries, 2021-03-03) to handle sparse directory entries in the index. Hence we can just set the requires-full-index to false for "write-tree".

The `p2000` tests demonstrate a ~96% execution time reduction for 'git write-tree' using a sparse index:

Test before after ----------------------------------------------------------------- 2000.78: git write-tree (full-v3) 0.34 0.33 -2.9% 2000.79: git write-tree (full-v4) 0.32 0.30 -6.3% 2000.80: git write-tree (sparse-v3) 0.47 0.02 -95.8% 2000.81: git write-tree (sparse-v4) 0.45 0.02 -95.6%

Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
---

* Modified the code to ensure prepare_repo_settings() is called only 
when inside a repository.

* Change 'write-tree on all' to just 'write-tree'.

* Have a baseline 'test_all_match git write-tree' before making any 
changes to the index.

* Add  'git status --porcelain=v2'.

* Ensuring that SKIP_WORKTREE files weren't materialized on disk by
using "test_path_is_missing".

* Use 'test_all_match' on the 'git update-index'.

Range-diff against v2:
1:  8873c79759 ! 1:  cfa43c6cc7 write-tree: integrate with sparse index
    @@ Commit message
     
      ## builtin/write-tree.c ##
     @@ builtin/write-tree.c: int cmd_write_tree(int argc, const char **argv, const char *cmd_prefix)
    - 	argc = parse_options(argc, argv, cmd_prefix, write_tree_options,
    - 			     write_tree_usage, 0);
    + 	};
      
    + 	git_config(git_default_config, NULL);
    ++	
    ++	if (the_repository->gitdir) {
     +	prepare_repo_settings(the_repository);
     +	the_repository->settings.command_requires_full_index = 0;
    -+	
    - 	ret = write_cache_as_tree(&oid, flags, tree_prefix);
    - 	switch (ret) {
    - 	case 0:
    ++	}
    ++
    + 	argc = parse_options(argc, argv, cmd_prefix, write_tree_options,
    + 			     write_tree_usage, 0);
    + 
     
      ## t/perf/p2000-sparse-operations.sh ##
     @@ t/perf/p2000-sparse-operations.sh: test_perf_on_all git checkout-index -f --all
    @@ t/t1092-sparse-checkout-compatibility.sh: test_expect_success 'grep sparse direc
      	test_cmp actual expect
      '
      
    -+test_expect_success 'write-tree on all' '
    ++test_expect_success 'write-tree' '
     +	init_repos &&
     +
    ++	test_all_match git write-tree &&
    ++
     +	write_script edit-contents <<-\EOF &&
     +	echo text >>"$1"
     +	EOF
     +
    ++	# make a change inside the sparse cone
     +	run_on_all ../edit-contents deep/a &&
    -+	run_on_all git update-index deep/a &&
    ++	test_all_match git update-index deep/a &&
     +	test_all_match git write-tree &&
    ++	test_all_match git status --porcelain=v2 &&
     +
    ++	# make a change outside the sparse cone
     +	run_on_all mkdir -p folder1 &&
     +	run_on_all cp a folder1/a &&
     +	run_on_all ../edit-contents folder1/a &&
    -+	run_on_all git update-index folder1/a &&
    -+	test_all_match git write-tree
    ++	test_all_match git update-index folder1/a &&
    ++	test_all_match git write-tree &&
    ++	test_all_match git status --porcelain=v2 &&
    ++	
    ++	# check that SKIP_WORKTREE files are not materialized
    ++	test_path_is_missing sparse-checkout/folder2/a &&
    ++	test_path_is_missing sparse-index/folder2/a
     +'
     +
     +test_expect_success 'sparse-index is not expanded: write-tree' '

 builtin/write-tree.c                     |  6 ++++
 t/perf/p2000-sparse-operations.sh        |  1 +
 t/t1092-sparse-checkout-compatibility.sh | 38 ++++++++++++++++++++++++
 3 files changed, 45 insertions(+)