sha1_file: do not access pack if unneeded

sha1_file: do not access pack if unneeded

Currently, regardless of the contents of the "struct object_info" passed
to sha1_object_info_extended(), that function always accesses the
packfile whenever it returns information about a packed object, since it
needs to populate "u.packed".

Add the ability to pass NULL, and use NULL-ness of the argument to
activate an optimization in which sha1_object_info_extended() does not
needlessly access the packfile. A subsequent patch will make use of this
optimization.

A similar optimization is not made for the cached and loose cases as it
would not cause a significant performance improvement.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>