cache-tree: fix inverted object existence check in cache_tree_fully_valid

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>