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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1092-sparse-checkout-compatibility.sh

@@ -2055,4 +2055,32 @@ test_expect_success 'grep sparse directory within submodules' '
 	test_cmp actual expect
 '
 
+test_expect_success 'write-tree on all' '
+	init_repos &&
+
+	write_script edit-contents <<-\EOF &&
+	echo text >>"$1"
+	EOF
+
+	run_on_all ../edit-contents deep/a &&
+	run_on_all git update-index deep/a &&
+	test_all_match git write-tree &&
+
+	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_expect_success 'sparse-index is not expanded: write-tree' '
+	init_repos &&
+
+	ensure_not_expanded write-tree &&
+
+	echo "test1" >>sparse-index/a &&
+	git -C sparse-index update-index a &&
+	ensure_not_expanded write-tree
+'
+
 test_done