pack: move clear_delta_base_cache(), packed_object_info(), unpack_entry()

pack: move clear_delta_base_cache(), packed_object_info(), unpack_entry()

Both sha1_file.c and packfile.c now need read_object(), so a copy of
read_object() was created in packfile.c.

This patch makes both mark_bad_packed_object() and has_packed_and_bad()
global. Unlike most of the other patches in this series, these 2
functions need to remain global.

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