cache-tree: fix inverted object existence check in cache_tree_fully_valid

The negation in front of the object existence check in
cache_tree_fully_valid() was lost in 062b914c84 (treewide: convert
users of `repo_has_object_file()` to `has_object()`, 2025-04-29),
turning `!repo_has_object_file(...)` into `has_object(...)` instead
of `!has_object(...)`.

This makes cache_tree_fully_valid() always report the cache tree as
invalid when objects exist (the common case), forcing callers like
write_index_as_tree() to call cache_tree_update() on every
invocation.  An odb_has_object() check inside update_one() avoids a
full tree rebuild, but the unnecessary call still pays the cost of
opening an ODB transaction and, in partial clones, a promisor remote
check.

Restore the missing negation and add a test that verifies write-tree
takes the cache-tree shortcut when the cache tree is valid.

Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: David Lin <davidlin@stripe.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -239,7 +239,7 @@ int cache_tree_fully_valid(struct cache_tree *it)
 	if (!it)
 		return 0;
 	if (it->entry_count < 0 ||
-	    odb_has_object(the_repository->objects, &it->oid,
+	    !odb_has_object(the_repository->objects, &it->oid,
 			   HAS_OBJECT_RECHECK_PACKED | HAS_OBJECT_FETCH_PROMISOR))
 		return 0;
 	for (i = 0; i < it->subtree_nr; i++) {

t/t0090-cache-tree.sh

@@ -278,4 +278,12 @@ test_expect_success 'switching trees does not invalidate shared index' '
 	)
 '
 
+test_expect_success 'cache-tree is used by write-tree when valid' '
+	test_commit use-valid &&
+
+	# write-tree with a valid cache-tree should skip cache_tree_update
+	GIT_TRACE2_PERF="$(pwd)/trace.output" git write-tree &&
+	test_grep ! region_enter.*cache_tree.*update trace.output
+'
+
 test_done