sha1_object_info: examine cached_object store too

sha1_object_info: examine cached_object store too

Cached object store was added in d66b37b (Add pretend_sha1_file()
interface. - 2007-02-04) as a way to temporarily inject some objects
to object store.

But only read_sha1_file() knows about this store. While it will return
an object from this store, sha1_object_info() will happily say
"object not found".

Teach sha1_object_info() about the cached store for consistency.

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