cache-tree: do not generate empty trees as a result of all i-t-a subentries

cache-tree: do not generate empty trees as a result of all i-t-a subentries

If a subdirectory contains nothing but i-t-a entries, we generate an
empty tree object and add it to its parent tree. Which is wrong. Such
a subdirectory should not be added.

Note that this has a cascading effect. If subdir 'a/b/c' contains
nothing but i-t-a entries, we ignore it. But then if 'a/b' contains
only (the non-existing) 'a/b/c', then we should ignore 'a/b' while
building 'a' too. And it goes all the way up to top directory.

Noticed-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>